Method and enb equipment for supporting seamless handover

ABSTRACT

The present application discloses a method and eNB equipment for supporting seamless handover. The method comprises the following steps of: receiving, by a target eNB, random access information or an RRC connection reconfiguration completion message from a UE; transmitting, by the target eNB, a data transmission stopping indication message to a source eNB; and, stopping, by the source eNB, transmitting downlink data to the UE, and/or stopping, by the source eNB, receiving uplink data from the UE. The present invention further provides several other methods and eNB equipments for supporting seamless handover. By the methods for supporting seamless handover provided by the present invention, the delay of data transmission and the unnecessary data transmission or unnecessary data monitoring of a source eNB can be avoided, the waste of resources and the power consumption can be reduced, and the missing and duplication transmission of data can be avoided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.17/322,675 filed on May 17, 2021, which is a continuation of U.S. patentapplication Ser. No. 17/066,371 filed on Oct. 8, 2020, which is acontinuation of U.S. patent application Ser. No. 16/090,140 filed onSep. 28, 2018, now U.S. Pat. No. 10,805,852 issued on Oct. 13, 2020,which is a 371 of International Application No. PCT/KR2017/003601 filedon Mar. 31, 2017, which claims priority to Chinese Patent ApplicationNo. 201610204551.9 filed on Apr. 1, 2016, Chinese Patent Application No.201610318090.8 filed on May 12, 2016, Chinese Patent Application No.201610840088.7 filed on Sep. 21, 2016 and Chinese Patent Application No.201610855465.4 filed on Sep. 27, 2016, the disclosures of which areherein incorporated by reference in their entirety.

BACKGROUND 1. Field

The present invention relates to wireless communication technologies,and in particular to a method and eNB equipment for supporting seamlesshandover.

2. Description of Related Art

The modern mobile communication increasingly tends to focus onmultimedia services that provide users with high-rate transmission. FIG.1 is a system architecture diagram showing the System ArchitectureEvolution (SAE). Wherein:

A User Equipment (UE) 101 is a terminal equipment used for receivingdata. An Evolved Universal Terrestrial Radio Access Network (E-UTRAN)102 is a radio access network in which a macro eNodeB/NodeB providingthe UE with an interface for accessing a radio network is included. AMobility Management Entity (MME) 103 is responsible for managing amovement context, a session context and security information for the UE.A Serving Gateway (SGW) 104 mainly functions to provide a user plane,and the MME 103 and the SGW 104 may be located in a same physicalentity. A Packet Data Network Gateway (PGW) 105 is responsible forbilling, lawful interception or more, and the PGW 105 and the SGW 104may also be located in a same physical entity. A Policy and ChargingRules Function Entity (PCRF) 106 provides Quality of Service (QoS)policy and charging rules. A Serving GPRS Support Node (SGSN) 108 is anetwork node equipment providing routing for data transmission in aUniversal Mobile Telecommunication System (UMTS). A Home SubscriberServer (HSS) 109 is a home subsystem of the UE, and is responsible forprotecting user information such as the current location of the UE, theaddress of a serving node, user security information, a packet datacontext of the UE, or more.

FIG. 2 shows a handover flow according to the present invention. Thismethod comprises the following steps.

Step 201: By a source eNB, a handover request message is transmitted toa target eNB.

Step 202: By the target eNB, a handover request acknowledge message istransmitted to the source eNB.

Step 203: By the source eNB, an RRC connection reconfiguration messageis transmitted to a UE. The source eNB stops transmitting downlink datato the UE. The source eNB stops receiving uplink data from the UE. Uponreceiving this message, the UE stops receiving the downlink datatransmitted by the source eNB and also stops transmitting the uplinkdata to the source eNB.

Step 204: By the source eNB, a Sequence Number (SN) status transfer istransmitted to the target eNB. The source eNB starts to forward data tothe target eNB.

Step 205: The UE is synchronized to the target eNB. The UE accesses to atarget cell via a Random Access Channel (RACH).

Step 206: By the UE, an RRC connection reconfiguration completionmessage is transmitted to the target eNB. The UE starts to transmit andreceive data in the target eNB.

Step 207: By the target eNB, a path handover request message istransmitted to an MME.

Step 208: By the MME, a path handover request response message istransmitted to the target eNB.

Step 209: By the target eNB, a UE context release message is transmittedto the source eNB.

From the step 203 to the step 206, the transmission of uplink anddownlink data of the UE is interrupted, and as a result, the delay ofdata transmission is caused. If the source eNB continues transmittingand receiving data after the step 203, the UE also continues receivingthe downlink data or transmitting the uplink data at the source eNBbetween the step 203 and the step 206, and the source eNB does not knowwhen to stop transmitting and receiving data until the step 209, thiswill cause unnecessary data transmission or unnecessary data monitoringof the source eNB, and result in unnecessary waste of resources andpower consumption.

In addition, in a downlink, the source eNB simultaneously transmits datato the UE and the target eNB after the step 203. Since an X2 interfacecannot ensure the sequential transmission of data, it is likely toresult in the missing or duplicated receipt of data packets, and the UEis unable to detect this case. For example, the source eNB forwards PDCPdata packets 10, 11, 12 carrying a Packet Data Convergence Protocol(PDCP) SN and three subsequent PDCP data packets (a, b, c) not carryinga PDCP SN to the target eNB. The source eNB informs the target eNB thatthe next SN to be used is 13. Since the PDCP data packets are nottransmitted sequentially, the order of the three PDCP data packetsreceived by the target eNB is (c, a, b). The correct order should be 10,11, 12, (a, 13), (b, 14), (c, 15). The actual data packets received bythe target eNB are 10, 11, 12, (c, 13), (a, 14), (b, 15). The datapackets received from the source eNB by the UE are 10, 11, 12, (a, 13).The UE informs the target eNB that the SN of the next PDCP data packetto be transmitted is 14. Therefore, the target eNB transmits datapackets (a, 14), (b, 15) . . . to the UE. Thus, the UE will receive thedata packet a twice, but the data packet c is missed. The PDCP of the UEis unable to detect that the data packet a is transmitted repeatedlybecause the SNs of the two data packets a are different.

In the prior art, the source eNB transmits an SN status transfer messageto the target eNB, and the source eNB freezes transmission and receipt.The source eNB informs the target eNB of a next PDCP SN to be used and acorresponding HFN through a DN Count in the SN status transfer message.After the UE is successfully connected to the target eNB, the UEtransmits a PDCP status report to the target eNB, so that the target eNBknows the SN of the next PDCP data packet to be transmitted to the UE.Furthermore, an HFN corresponding to the next PDCP data packet is theHFN in the DL count or is smaller 1 than the HFN in the DL count. Thisis because that the data packet corresponding to the next PDCP SN to beused has not yet been transmitted to the UE (the source eNB has frozenthe transmission and receive status when transmitting the SN statustransfer message). During the enhanced handover process, upontransmitting the SN status transfer message to the target eNB, thesource eNB continues transmitting downlink data to the UE, andmeanwhile, the source eNB forwards data to the target eNB. Since somedata packets have been received by the UE at the source eNB, these datapackets are not to be transmitted repeatedly by the target eNB. Afterthe UE is successfully connected to the target eNB, the target eNB willreceive a PDCP status report from the UE, so that the target eNB canknow the PDCP SN from which the transmission of data packets to the UEis started. However, the target eNB does not know an HFN correspondingto the next expected PDCP SN received from the UE.

In an uplink, during the existing handover process, the source eNBinforms the target eNB of a receive status of an uplink PDCP and anuplink count through an SN status transfer message, and the source eNBstops transmitting downlink data and receiving uplink data. The targeteNB knows an uplink transmission status from the source eNB to the SGW.The target eNB informs the UE of the uplink receive status of the sourceeNB, so that the UE starts the transmission from the next PDCP datapacket that has not been received by the source eNB. If the source eNBcontinues transmitting and receiving data after the step 203, and afterthe source eNB transmits an SN status transfer message to the targeteNB, the UE still transmits the uplink data, and the source eNB stillreceives the uplink data from the UE. After the UE is successfullysynchronized to the target eNB or the RRC reconfiguration is successful,the target eNB does not know the uplink receive status of the source eNBin receiving data from the UE, and also does not know the uplinktransmission status from the source eNB to the SGW. The uplink receivestatus in the SN status transfer message received by the target eNB inthe step 204 will be no longer valid, and as a result, the seamlesstransmission of uplink data cannot be ensured.

SUMMARY

The present invention provides a method and eNB equipment for supportingseamless handover, in order to solve the above-mentioned problems of thedelay of data transmission, the unnecessary data transmission orunnecessary data monitoring of a source eNB, and the missing andduplication transmission of uplink and downlink data in the handoverprocess.

The present invention provides a method for supporting seamlesshandover, comprising the following steps of:

transmitting, by a source eNB, a handover request message to a targeteNB;

receiving a handover request acknowledge message transmitted by thetarget eNB;

transmitting an RRC connection reconfiguration message to a UE,continuing transmitting downlink data to the UE, and continuingreceiving uplink data transmitted by the UE;

transmitting a Sequence Number (SN) status transfer to the target eNB,and starting to forward data to the target eNB;

receiving a data transmission stopping indication transmitted by thetarget eNB, and by the source eNB, stopping transmitting the downlinkdata to the UE and stopping receiving the uplink data from the UE; and

receiving a UE context release message transmitted by the target eNB.

Preferably, after the UE is synchronized to the target eNB, the targeteNB transmits a data transmission stopping indication to the source eNB.

The data transmission stopping indication is transmitted by the targeteNB after the UE is synchronized to the target eNB.

Preferably, the method further comprises the following step of:

transmitting, by the source eNB, a second SN status transfer message tothe target eNB.

Preferably, the second SN status transfer message contains:

a receive status of an uplink Packet Data Convergence Protocol SequenceNumber (PDCP SN) and/or Hyper-Frame Number (HFN); or

a transmission status of a downlink PDCP SN and/or HFN.

Preferably, the second SN status transfer message and the SN statustransfer are a same message or different messages; and

when the second SN status transfer message is transmitted, it isindicated that the source eNB has frozen the transmission and receivestatus.

Preferably, the target eNB detects data that is not to be transmitted tothe UE in at least one of the following ways:

way 1: according to a transmission status of a PDCP SN and/or HFNreceived from the source eNB in the second status transfer message, thetarget eNB discards duplicated data and directly transmits data that hasbeen not received by the UE, where the duplicated data refers to datathat is transmitted to the UE and forwarded to the target eNBsimultaneously by the source eNB;

way 2: according to a PDCP status report received from the UE, thetarget eNB detects duplicated data that is not to be transmitted to theUE, then discards the duplicated data and directly transmits data thathas been not received by the UE; and

way 3: in combination with the second SN status transfer message and thePDCP status report received from the UE, the target eNB detectsduplicated data that is not to be transmitted to the UE, then discardsthe duplicated data and directly transmits data that has been notreceived by the UE, the second SN status transfer message containing thetransmission status of the PDCP SN and/or HFN.

Preferably, after transmitting, by the source eNB, an RRC connectionreconfiguration message to a UE or transmitting, by the source eNB, anSN status transfer to the target eNB, the method further comprises thefollowing step of:

feeding, by the source eNB, an uplink data receive status back to theUE.

Preferably, the step of transmitting, by the source eNB, a second SNstatus transfer message to the target eNB further comprises:

transmitting, by the source eNB, an uplink receive status after stoppingreceiving the uplink data to the target eNB; or

transmitting, by the source eNB, an uplink receive status after stoppingreceiving the uplink data and an uplink count to the target eNB.

Preferably, after transmitting, by the source eNB, an SN status transferto the target eNB and before transmitting, by the source eNB, a secondSN status transfer message to the target eNB, or, after transmitting, bythe source eNB, an SN status transfer to the target eNB and beforereceiving a data transmission stopping indication transmitted by thetarget eNB, the method further comprises the following step of:

by the source eNB, transmitting the sequentially received uplink datapackets to an SGW, and forwarding other data packets to the target eNB.

Preferably, the process of stopping, by the UE, transmitting uplink datato the source eNB further comprises:

according to the received uplink receive status and in combination withthe data that is forwarded by the source eNB and received from thesource eNB, transmitting, by the target eNB, the updated uplink receivestatus to the UE; and

according to an end marker, determining, by the target eNB, that thesource eNB has forwarded the data.

Preferably, in the step of forwarding, by the source eNB, data to thetarget eNB, the data is:

an order of forwarded data packets identified by an SN in a GTP-Uheader; or

PDCP data packets containing an SN, meanwhile, the PDCP data packetscontaining an SN are transmitted to the UE; or

data packets containing a defined Frame Protocol (FP), where a dataportion of the FP contains the forwarded PDCP data packets, and a packetheader of the FP contains a sequence number of the data packets; or

data packets containing a PDCP SN, where a PDCP SN used by the firstdata packet is a set particular value or set to be dummy; or

an estimated number of transmitted data packets after the number of thePDCP data packets is estimated by the source eNB.

Preferably, after transmitting, by the source eNB, an RRC connectionreconfiguration message to the UE, the method further comprises:

by the UE, continuing receiving the downlink data transmitted by thesource eNB, and continuing transmitting the uplink data to the sourceeNB; and

stopping receiving the downlink data from the source eNB, and stoppingtransmitting the uplink data to the source eNB, after the UE issynchronized to the target eNB.

An eNB equipment is provided, comprising: a first transmitting module, afirst processing module and a first receiving module, wherein:

the first transmitting module is configured to: transmit a handoverrequest message to a target eNB; transmit an RRC connectionreconfiguration message to a UE, and continue transmitting downlink datato the UE; and, transmit an SN status transfer to the target eNB, andstart to forward data to the target eNB;

the first receiving module is configured to: continue receiving uplinkdata from the UE; receive a data transmission stopping indicationtransmitted by the target eNB and indicate the first processing moduleto perform processing; and, receive a UE context release messagetransmitted by the target eNB; and

the first processing module is configured to, under an indication fromthe receiving module, control the first transmitting module to stoptransmitting the downlink data to the UE and control the first receivingmodule to stop receiving the uplink data from the UE.

A method for supporting seamless handover is provided, comprising thefollowing steps of:

transmitting, by a target eNB, a handover request acknowledge messageupon receiving a handover request message transmitted by a source eNB;

receiving a Sequence Number (SN) status transfer transmitted by thesource eNB after transmitting an RRC connection reconfiguration messageto a UE;

transmitting, by the target eNB, a data transmission stopping indicationto the source eNB after the UE is synchronized to the target eNB;

receiving an RRC connection reconfiguration completion messagetransmitted by the UE, and transmitting a path handover request messageto an MME; and

receiving a path handover request response message transmitted by theMME, and transmitting a UE context release message to the source eNB.

Preferably, the method further comprises the following step of:

receiving a second SN status transfer message transmitted by the sourceeNB; and

detecting, by the target eNB, data that is not to be transmitted to theUE in at least one of the following ways:

way 1: according to a transmission status of a PDCP SN and/or HFNreceived from the source eNB in the second SN status transfer message,the target eNB discards duplicated data and directly transmits data thathas been not received by the UE, the duplicated data referring to datathat is transmitted to the UE and forwarded to the target eNBsimultaneously by the source eNB;

way 2: according to a PDCP status report received from the UE, thetarget eNB detects duplicated data that is not to be transmitted to theUE, then discards the duplicated data and directly transmits data thathas been not received by the UE; and

way 3: in combination with the second SN status transfer message and thePDCP status report received from the UE, the target eNB detectsduplicated data that is not to be transmitted to the UE, then discardsthe duplicated data and directly transmits data that has been notreceived by the UE, the second SN status transfer message containing thetransmission status of the PDCP SN and/or HFN.

Preferably, after the UE is synchronized to the target eNB, the methodfurther comprises:

by the UE, stopping receiving downlink data from the source eNB, andstopping transmitting uplink data to the source eNB; and

the process of stopping, by the UE, transmitting uplink data to thesource eNB further comprises:

according to the uplink receive status fed back by the source eNB and incombination with the data that is forwarded by the source eNB andreceived from the source eNB, transmitting, by the target eNB, theupdated uplink receive status to the UE; and

according to an end marker, determining, by the target eNB, that thesource eNB has forwarded the data.

An eNB equipment is provided, comprising: a second transmitting moduleand a second receiving module, wherein:

the second receiving module is configured to: receive a handover requestmessage transmitted by a source eNB; receive an RRC connectionreconfiguration message transmitted to a UE by the source eNB; receivean RRC connection reconfiguration completion message transmitted by theUE; and receive a path handover request response message transmitted byan MME; and

the second transmitting module is configured to: transmit a handoverrequest acknowledge message to the source eNB; transmit, by a targeteNB, a data transmission stopping indication to the source eNB after theUE is synchronized to the target eNB; transmit a path request message tothe MME; and transmit a UE context release message to the source eNB.

A method for supporting seamless handover is provided, comprising thefollowing steps of:

by a source eNB, transmitting a handover request message to a targeteNB, and receiving a handover request acknowledge message transmitted bythe target eNB;

transmitting an RRC connection reconfiguration message to a UE,continuing transmitting downlink data to the UE, and continuingreceiving uplink data transmitted by the UE;

transmitting a Sequence Number (SN) status transfer to the target eNB,and starting to forward data to the target eNB;

receiving a data transmission stopping indication transmitted by thetarget eNB after receiving an RRC connection reconfiguration completionmessage, and stopping transmitting the downlink data to the UE; and

receiving a UE context release message transmitted by the target eNB.

Preferably, the method further comprises the following steps of:

stopping, by the UE, transmitting the uplink data to the source eNBafter the UE is synchronized to the target eNB; or

by the UE, transmitting an RRC connection reconfiguration completionmessage to the target eNB, and stopping transmitting the uplink data tothe source eNB.

Preferably, the method further comprises the following steps of:

by the source eNB, receiving an uplink data stopping receivingindication transmitted by the target eNB after receiving synchronizationinformation transmitted by the UE, and stopping receiving the uplinkdata from the UE; and

when this step is not executed and by the source eNB, stopping receivingthe uplink data from the UE upon receiving the data transmissionstopping indication message.

Preferably, the method further comprises the following step of:

transmitting, by the source eNB, a second SN status transfer message tothe target eNB, this message containing a receive status of an uplinkPDCP SN and/or HFN.

Preferably, after transmitting, by the source eNB, an RRC connectionreconfiguration message to a UE or transmitting, by the source eNB, anSN status transfer to the target eNB, the method further comprises thefollowing step of:

feeding, by the source eNB, an uplink data receive status back to theUE.

Preferably, the step of transmitting, by the source eNB, a second SNstatus transfer message to the target eNB further comprises:

transmitting, by the source eNB, an uplink receive status after stoppingreceiving the uplink data to the target eNB; or

transmitting, by the source eNB, an uplink receive status after stoppingreceiving the uplink data and an uplink count to the target eNB.

Preferably, after transmitting, by the source eNB, an SN status transferto the target eNB and before transmitting, by the source eNB, a secondSN status transfer message to the target eNB, or, after transmitting, bythe source eNB, an SN status transfer to the target eNB and beforereceiving a data transmission stopping indication transmitted by thetarget eNB, the method further comprises the following step of:

by the source eNB, transmitting the sequentially received uplink datapackets to an SGW, and forwarding other data packets to the target eNB.

Preferably, the process of stopping, by the UE, transmitting uplink datato the source eNB further comprises:

according to the received uplink receive status and in combination withthe data that is forwarded by the source eNB and received from thesource eNB, transmitting, by the target eNB, the updated uplink receivestatus to the UE; and

by the target eNB, determining, according to an end marker, that thesource eNB has forwarded the data, and transmitting a data transmissionstopping indication to the source eNB.

Preferably, in the step of forwarding, by the source eNB, data to thetarget eNB, the data is:

an order of forwarded data packets identified by an SN in a GTP-Uheader; or

PDCP data packets containing an SN, meanwhile, PDCP data packetscontaining an SN are transmitted to the UE; or

data packets containing a defined Frame Protocol (FP), where a dataportion of the FP contains the forwarded PDCP data packets and a packetheader of the FP contains a sequence number of the data packets; or

data packets containing a PDCP SN, where a PDCP SN used by the firstdata packet is a set particular value or set to be dummy; or

an estimated number of transmitted data packets after the number of thePDCP data packets is estimated by the source eNB.

An eNB equipment is provided, comprising: a third transmitting module, athird processing module and a third receiving module, wherein:

the third transmitting module is configured to: transmit a handoverrequest message to a target eNB; transmit an RRC connectionreconfiguration message to a UE, and continue transmitting downlink datato the UE; and, transmit an SN status transfer to the target eNB, andstart to forward data to the target eNB;

the third receiving module is configured to: receive a handover requestacknowledge message transmitted by the target eNB, and continuereceiving uplink data from the UE; receive a data transmission stoppingindication transmitted by the target eNB after receiving an RRCconnection reconfiguration completion message and indicates the thirdprocessing module to perform processing; and, receive a UE contextrelease message transmitted by the target eNB; and

the third processing module is configured to, under an indication fromthe third receiving module, control the transmitting module to stoptransmitting the downlink data to the UE and control the third receivingmodule to stop receiving the uplink data from the UE.

A method for supporting seamless handover is provided, comprising thefollowing steps of:

transmitting, by a target eNB, a handover request acknowledge messageupon receiving a handover request message transmitted by a source eNB;

receiving a Sequence Number (SN) status transfer transmitted by thesource eNB after transmitting an RRC connection reconfiguration messageto a UE;

receiving an RRC connection reconfiguration completion messagetransmitted by the UE, and transmitting a data transmission stoppingindication to the source eNB;

transmitting a path handover request message to an MME; and

receiving a path handover request response message transmitted by theMME, and transmitting a UE context release message to the source eNB.

Preferably, the method further comprises the following step of:

stopping, by the UE, transmitting uplink data to the source eNB afterthe UE is synchronized to a target eNB; or, by the UE, transmitting anRRC connection reconfiguration completion message to the target eNB, andstopping transmitting uplink data to the source eNB; and

the process of stopping, by the UE, transmitting uplink data to thesource eNB further comprises:

according to the uplink receive status fed back by the source eNB and incombination with the data that is forwarded by the source eNB andreceived from the source eNB, transmitting, by the target eNB, theupdated uplink receive status to the UE; and

by the target eNB, determining, according to an end marker, that thesource eNB has forwarded the data, and transmitting a data transmissionstopping indication to the source eNB.

An eNB equipment is provided, comprising: a fourth transmitting moduleand a fourth receiving module, wherein:

the fourth transmitting module is configured to: transmit a handoverrequest acknowledge message to a source eNB; transmit a datatransmission stopping indication to the source eNB; transmit a pathhandover request message to an MME; and transmit a UE context releasemessage to the source eNB; and

the fourth receiving module is configured to: receive a handover requestmessage transmitted by the source eNB; receive an RRC connectionreconfiguration message transmitted to the UE by the source eNB; receivean RRC connection reconfiguration completion message transmitted by theUE; and receive a path handover request response message transmitted bythe MME.

A method for supporting seamless handover for dual connectivity isprovided, comprising the following steps of:

receiving, by a target eNB, an eNB addition request message transmittedby a Macrocell eNB (MeNB);

transmitting, by the target eNB, an eNB addition request acknowledgemessage to the MeNB;

if resources for the target eNB are allocated successfully,transmitting, by the MeNB, an eNB release request message to a sourceeNB, continuing transmitting uplink and downlink data by the source eNB,and forwarding, by the source eNB, data to the target eNB;

by the MeNB, transmitting an RRC connection reconfiguration message to aUE, continuing transmitting downlink data to the source eNB, andforwarding the downlink data to the target eNB;

by the UE, receiving the RRC connection reconfiguration message, notdeleting or reconfiguring a configuration for the source eNB, continuingtransmitting uplink and downlink data between the UE and the source eNB,and transmitting an RRC connection reconfiguration completion message tothe MeNB;

transmitting, by the MeNB, an SeNB reconfiguration completion message tothe target eNB;

synchronizing the UE to the target eNB, and stopping transmitting uplinkand downlink data between the UE and the source eNB;

transmitting a data transmission stopping indication to the MeNB by thetarget eNB, and by the MeNB, stopping transmitting downlink data to thesource eNB upon receiving the message, and by the MeNB, transmitting thedata transmission stopping indication to the source eNB;

transmitting, by the source eNB, an SN status transfer to the MeNB; and

transmitting, by the MeNB, the SN status transfer to the target eNB.

It can be seen from the technical solutions that, by the methods forsupporting seamless handover provided by the present invention, thedelay of data transmission and the unnecessary data transmission orunnecessary data monitoring of a source eNB can be avoided, the waste ofresources and the power consumption can be reduced, and the missing andduplicated transmission of data can be avoided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an architecture diagram of an existing SAE systemarchitecture;

FIG. 2 is a flowchart of an existing handover;

FIG. 3 is a schematic diagram of a first method for supporting seamlesshandover according to the present invention;

FIG. 4 is a schematic diagram of a second method for supporting seamlesshandover according to the present invention;

FIG. 5 is a schematic diagram of a third method for supporting seamlesshandover according to the present invention;

FIG. 6 is a schematic diagram of an embodiment for dual connectivity ofthe first method for supporting seamless handover according to thepresent invention;

FIG. 7 is a schematic diagram of a fourth method for supporting seamlesshandover according to the present invention;

FIG. 8 is a schematic flowchart of a fifth method for supportingseamless handover according to the present invention;

FIG. 9 is a schematic diagram of an embodiment for dual connectivity ofthe fourth and fifth methods for supporting seamless handover accordingto the present invention;

FIG. 10 is a schematic diagram of a sixth method for supporting seamlesshandover according to the present invention;

FIG. 11 is a schematic diagram of an embodiment for dual connectivity ofthe sixth method for supporting seamless handover according to thepresent invention.

DETAILED DESCRIPTION

To make the objectives, technical means and advantages of the presentapplication clearer, the present application will be further describedbelow in details with reference to the accompanying drawings.

FIG. 3 shows a first method for supporting seamless handover accordingto the present invention. This method comprises the following steps.

Step 301: By a source eNB, a handover request message is transmitted toa target eNB.

Step 302: By the target eNB, a handover request acknowledge message istransmitted to the source eNB.

Step 303: By the source eNB, an RRC connection reconfiguration messageis transmitted to a UE. The source eNB continues transmitting downlinkdata to the UE. The source eNB continues receiving uplink data from theUE. Upon receiving this message, the UE continues receiving the downlinkdata transmitted by the source eNB, and continues transmitting theuplink data to the source eNB.

In order to solve the problem in the uplink data transmission in thepresent invention, one corresponding method is that the source eNBcontinues feeding an uplink data receive status back to the UE afterthis step or after step 304. Thus, the UE knows which data packets havebeen already received from the UE by the source eNB. Accordingly, upontransmitting a message in step 307, the UE knows a data packet startingfrom which the data packet should be transmitted to the target eNB.

In order to solve the problem in the uplink data transmission in thepresent invention, a second method is that the source eNB transmits anuplink receive status after stopping receiving the uplink data to thetarget eNB via step 306 a. Thus, the target eNB can accurately know anuplink receive status of the source eNB after the UE stops transmittingthe uplink data, so that the target eNB can inform the UE of this, andthe UE then transmits data packets that have not been received by thesource eNB to the target eNB. Corresponding to this method, morepreferably, step 304 can be omitted and not be executed. The source eNBtransmits an uplink count after stopping receiving the uplink data tothe target eNB via step 306 a. Thus, the target eNB knows a PDCP packetstarting from which the data packet should be transmitted to an SGW,according to the received uplink count. Corresponding to this method,more preferably, after the step 304 and before the step 306 or 306 a,the source eNB can directly transmit the sequentially received uplinkdata packets to the SGW, and forward other data packets to the targeteNB.

Step 304: By the source eNB, a Sequence Number (SN) status transfer istransmitted to the target eNB.

In this step, the source eNB does not freeze the transmission andreceive status.

The source eNB starts to forward data to the target eNB.

In order to solve the problem in the missing or duplicated transmissionof downlink data in an enhanced handover process, there are thefollowing several methods.

Method 1: An order of forwarded data packets is identified by an SN in aGTP-U header. The target eNB knows a sequential order of receiving PDCPdata packets according to the SN in the GTP-U header, then identifiesthe first data packet without a PDCP SN according to the SN to be usedby the next PDCP received from the source eNB, and successivelyidentifies subsequent data packets. Thus, upon receiving the next PDCPSN to be transmitted from the UE, the target eNB can correctly transmitthe next PDCP data packet that has not been received by the UE. Even ifthe X2 interface cannot realize sequential transmission, according tothe SN in the GTP-U header, the target eNB still knows the correct orderof the data packets transmitted by the source eNB, so that the correctPDCP data packet is marked with the next PDCP SN, thereby avoiding themissing or duplicated transmission of a data packet.

Method 2: Only the data is forwarded in this step, without transmittingthe SN status transfer message to the target eNB. The source eNBcontains an SN in all PDCP data packets forwarded to the target eNB, andthe data packets carrying the SN are also to be transmitted to the UE.In the step 306 a, the source eNB informs the target eNB of the PDCPuplink receive status and uplink and downlink counts of the source eNB.Thus, the target eNB knows a downlink transmission status on the sourceside when the source eNB stops transmitting the downlink data, and anuplink receive status when the source eNB stops receiving the uplinkdata packets and a transmission status of the uplink data transmitted bythe source eNB to the SGW. Meanwhile, the problem in the uplink anddownlink data transmission in the present invention is solved, and nomissing and duplicated transmission of data is ensured.

Method 3: A Frame Protocol (FP) is defined in the data packets forwardedto the target eNB by the source eNB. A data portion of the FP containsthe forwarded PDCP data packets, and a packet header of the FP containsa sequence number of the forwarded data packets. The target eNB knows asequential order of receiving PDCP data packets according to the SN inthe FP header, then identifies the first data packet without the PDCP SNaccording to the SN to be used by the next PDCP received from the sourceeNB, and successively identifies subsequent data packets. Thus, uponreceiving the next PDCP SN to be transmitted from the UE, the target eNBcan correctly transmit the next PDCP data packet that has not beenreceived by the UE. Even if the X2 interface cannot realize sequentialtransmission, according to the SN in the FP header, the target eNB stillknows the correct order of the data packets transmitted by the sourceeNB, so that the correct PDCP data packet is marked with the next PDCPSN, thereby avoiding the missing or duplicated transmission of a datapacket.

Method 4: All data packets forwarded to the target eNB by the source eNBcontain a PDCP SN. The next PDCP SN to be used, contained in the datapackets transmitted to the target eNB by the source eNB, is dummy. Uponreceiving the forwarded data packets, the target eNB ignores thereceived next PDCP SN to be used if it is found that all the forwardeddata packets contain the SN. Or, the next PDCP SN to be used, containedin the data packets transmitted to the target eNB by the source eNB, isa particular value. Upon receiving the SN status transfer message, thetarget eNB ignores the received next PDCP SN to be used if it is foundthat the next PDCP SN to be used is a particular value.

Method 5: The source eNB estimates the number of PDCP data packetstransmitted to the UE after receiving the SN status transfer message.Upon transmitting a corresponding number of data packets, the source eNBstops transmitting data to the UE. When forwarding a correspondingnumber of data packets to the target eNB, the source eNB contains a PDCPSN in the data packets, but does not contain a PDCP SN in the subsequentdata packets. The target eNB uses the received next PDCP SN to be usedto represent the received data packet without a PDCP SN.

Step 305: The UE is synchronized to the target eNB. The UE accesses to atarget cell via a Random Access Channel (RACH). The UE stops receivingthe downlink data from the source eNB, and stops transmitting the uplinkdata at the source eNB.

In order to solve the problem in the uplink data transmission in thepresent invention, a third method is as follows: according to the uplinkreceive status received in the message of the step 304 and incombination with the uplink data that is forwarded by the source eNB andreceived from the source eNB, the target eNB updates the uplink receivestatus and then transmits the updated uplink receive status to the UE.If the target eNB still continues receiving data forwarded from thesource eNB after the RACH is successful, the target eNB forms a newuplink receive status upon receiving all forwarded data from the sourceeNB. The target eNB knows that the source eNB has finished dataforwarding according to an end marker. Corresponding to the thirdmethod, steps 306 and 306 a may not be executed.

Step 306: By the target eNB, a data transmission stopping indication istransmitted to the source eNB. The message for transmitting the datatransmission stopping indication contains a new eNB UE X2AP (X2Application Protocol) ID and an old eNB UE X2AP ID of the UE. Uponreceiving this message, the source eNB stops transmitting the downlinkdata to the UE. The source eNB stops receiving the uplink data from theUE.

The data forwarded to the target eNB by the source eNB can contain PDCPdata packets containing an SN, and PDCP data packets containing no SN.The source eNB informs the target eNB of the next PDCP SN to be used viastep 306 a.

Through this process, the source eNB can timely know that the UE stopsreceiving and transmitting the data at the source eNB, so that thesource eNB does not need an idle air interface for data transmission oruplink receipt.

The method provided by the present invention is described by taking anX2 handover process as example. If the handover is an S1 handover, thetarget eNB transmits the data transmission stopping indication to thesource eNB by a Mobility Management Entity (MME). The S1 message fortransmitting the data transmission stopping indication contains an eNBeNB UE S1AP (S1 Application Protocol) ID and an MME UE S1AP ID.

Step 306 a: By the source eNB, a second SN status transfer message istransmitted to the target eNB. This message contains a receive status ofan uplink Packet Data Convergence Protocol Sequence Number (PDCP SN)and/or Hyper-Frame Number (HFN). The message contains a transmissionstatus of a downlink PDCP SN and/or HFN. The message contains a receivestatus of an uplink PDCP SDU, an uplink count and a downlink count. Thesecond SN status transfer message and the SN status transfer in the step304 can be a same message or different messages. When the second SNstatus transfer message is transmitted, it is indicated that the sourceeNB has frozen the transmission and receive status.

In the method provided by the present invention, this step is optionaland may or may not be executed.

Step 307: By the UE, an RRC connection reconfiguration completionmessage is transmitted to the target eNB. After the step 303, the sourceeNB simultaneously transmits data to the UE and forwards data to thetarget eNB. Therefore, a part of the forwarded data received from thesource eNB by the target eNB may be already received by the UE. Thereare following three ways for the target eNB to detect data that is notto be transmitted to the UE.

Way 1: According to a transmission status of a PDCP SN and/or HFNreceived from the source eNB in the step 306 a, the target eNB discardsduplicated data and directly transmits data that has been not receivedby the UE. The duplicated data refers to data that is transmitted to theUE and forwarded to the target eNB simultaneously by the source eNB.

Way 2: According to a PDCP status report received from the UE, thetarget eNB detects duplicated data that is not to be transmitted to theUE, then discards the duplicated data and directly transmits data thathas been not received by the UE. The target eNB transmits data packetsto the UE, starting from a data packet having the next PDCP SN expectedby the UE in the PDCP status report.

Way 3: In combination with the transmission status of the PDCP SN and/orHFN received in the step 306 a and the PDCP status report received fromthe UE, the target eNB detects duplicated data that is not to betransmitted to the UE, then discards the duplicated data and directlytransmits data that has been not received by the UE. In other words, incombination with the second SN status transfer message and the PDCPstatus report received from the UE, the target eNB detects duplicateddata that is not to be transmitted to the UE, then discards theduplicated data and directly transmits data that has been not receivedby the UE, the second SN status transfer message containing thetransmission status of the PDCP SN and/or HFN.

The target eNB knows the next data packet that is expected to bereceived according to the receive status of uplink PDCP data packetsreceived in the step 306 a, and the target eNB transmits thisinformation to the UE. Thus, the UE transmits, to the target eNB, thedata packet starting from the next data packet that has not beenreceived from the network side. The target eNB knows the next datapacket to be transmitted to the SGW according to the uplink countreceived from the source eNB in the step 306 a, so that a data packet isavoided from being repeatedly transmitted to the SGW or being missed.

According to the uplink receive status received from the target eNB andin combination with the data transmitted by the UE at the source eNB andthe feedback received from the source eNB, the UE transmits uplink datapackets to the target eNB starting from the next data packet that hasnot been received by the source eNB.

Step 308: By the target eNB, a path handover request message istransmitted to an MME.

Step 309: By the MME, a path handover request response message istransmitted to the target eNB.

Step 310: By the target eNB, a UE context release message is transmittedto the source eNB.

Now, the first method for supporting seamless handover according to thepresent invention has been described. By this method, the unnecessarydata transmission or unnecessary data monitoring of the source eNB canbe avoided. Actually, the UE has stopped the downlink receipt and uplinktransmission at the source eNB, so the air interface resources and powerloss are saved. By this method, the continuous transmission of uplinkand downlink data can be ensured, and the missing or duplicationtransmission of data can be avoided.

FIG. 4 shows a second method for supporting seamless handover accordingto the present invention. This method comprises the following steps.

Step 401: By a source eNB, a handover request message is transmitted toa target eNB.

Step 402: By the target eNB, a handover request acknowledge message istransmitted to the source eNB.

Step 403: By the source eNB, an RRC connection reconfiguration messageis transmitted to a UE. The source eNB continues transmitting downlinkdata to the UE. The source eNB continues receiving uplink data from theUE. Upon receiving this message, the UE continues receiving the downlinkdata transmitted by the source eNB, and continues transmitting theuplink data to the source eNB.

In order to solve the problem in the uplink data transmission in thepresent invention, one corresponding method is that the source eNBcontinues feeding an uplink data receive status back to the UE afterthis step or after step 404. Thus, the UE knows which data packets havebeen already received from the UE by the source eNB. Accordingly, upontransmitting a message in step 407, the UE knows a data packet startingfrom which the data packet should be transmitted to the target eNB.

In order to solve the problem in the uplink data transmission in thepresent invention, a second method is that the source eNB transmits anuplink receive status after stopping receiving the uplink data to thetarget eNB via step 406 a or step 408 a. Thus, the target eNB canaccurately know an uplink receive status of the source eNB after the UEstops transmitting the uplink data, so that the target eNB can informthe UE of this, and the UE then transmits data packets that have notbeen received by the source eNB to the target eNB. Corresponding to thismethod, more preferably, step 404 can be omitted and not executed. Thesource eNB transmits an uplink count after stopping receiving the uplinkdata to the target eNB via step 406 a or step 408 a. Thus, the targeteNB knows a PDCP packet starting from which the data is transmitted toan SGW, according to the received uplink count. Corresponding to thismethod, more preferably, after the step 404 and before the step 406 or406 a, or the step 408 or 408 a, the source eNB can directly transmitthe sequentially received uplink data packets to the SGW, and forwardother data packets to the target eNB.

Step 404: By the source eNB, a Sequence Number (SN) status transfer istransmitted to the target eNB.

In this step, the source eNB does not freeze the transmission andreceive status.

The source eNB starts to forward data to the target eNB.

In order to solve the problem in the missing or duplication transmissionof downlink data in an enhanced handover process, there are thefollowing several methods.

Method 1: An order of forwarded data packets is identified by an SN in aGTP-U header. The target eNB knows a sequential order of receiving PDCPdata packets according to the SN in the GTP-U header, then identifiesthe first data packet without a PDCP SN according to the SN to be usedby the next PDCP received from the source eNB, and successivelyidentifies subsequent data packets. Thus, upon receiving the next PDCPSN to be transmitted from the UE, the target eNB can correctly transmitthe next PDCP data packet that has not been received by the UE. Even ifthe X2 interface cannot realize sequential transmission, according tothe SN in the GTP-U header, the target eNB still knows the correct orderof the data packets transmitted by the source eNB, so that the correctPDCP data packet is marked with the next PDCP SN, thereby avoiding themissing or duplication transmission of the data packets.

Method 2: Only the data is forwarded in this step, without transmittingthe SN status transfer message to the target eNB. The source eNBcontains an SN in all PDCP data packets forwarded to the target eNB, andthe data packets carrying the SN are also to be transmitted to the UE.The source eNB informs the target eNB of the receive status of theuplink PDCP data packets and an uplink count in step 406 a, and informsthe target eNB of a downlink count via step 408 a. Or, the source eNBinforms the target eNB of the PDCP uplink receive status and uplink anddownlink count of the source eNB via step 408 a. Thus, the target eNBknows a downlink transmission status on the source side when the sourceeNB stops transmitting the downlink data, and an uplink receive statuswhen the source eNB stops receiving the uplink data packets, and atransmission status of the uplink data transmitted by the source eNB tothe SGW. Meanwhile, the problem in the uplink and downlink datatransmission in the present invention is solved, and no missing andduplication transmission of data is ensured.

Method 3: A FP is defined in the data packets forwarded to the targeteNB by the source eNB. A data portion of the FP contains the forwardedPDCP data packets, and a packet header of the FP contains a sequencenumber of the forwarded data packets. The target eNB knows a sequentialorder of receiving PDCP data packets according to the SN in the FPheader, then identifies the first data packet without the PDCP SNaccording to the SN to be used by the next PDCP received from the sourceeNB, and successively identifies subsequent data packets. Thus, uponreceiving the next PDCP SN to be transmitted from the UE, the target eNBcan correctly transmit the next PDCP data packet that has not beenreceived by the UE. Even if the X2 interface cannot realize sequentialtransmission, according to the SN in the FP header, the target eNB stillknows the correct order of the data packets transmitted by the sourceeNB, so that the correct PDCP data packet is marked with the next PDCPSN, thereby avoiding the missing or duplication transmission of the datapackets.

Method 4: All data packets forwarded to the target eNB by the source eNBcontain a PDCP SN. The next PDCP SN to be used, contained in the datapackets transmitted to the target eNB by the source eNB, is dummy. Uponreceiving the forwarded data packets, the target eNB ignores thereceived next PDCP SN to be used if it is found that all the forwardeddata packets contain the SN. Or, the next PDCP SN to be used containedin the data packets transmitted to the target eNB by the source eNB is aparticular value. Upon receiving the SN status transfer message, thetarget eNB ignores the received next PDCP SN to be used if it is foundthat the next PDCP SN to be used is a particular value.

Method 5: The source eNB estimates the number of PDCP data packetstransmitted to the UE after receiving the SN status transfer message.Upon transmitting a corresponding number of data packets, the source eNBstops transmitting data to the UE. When forwarding a correspondingnumber of data packets to the target eNB, the source eNB contains a PDCPSN in the data packets, but does not contain a PDCP SN in the subsequentdata packets. The target eNB uses the received next PDCP SN to be usedto represent the received data packet without a PDCP SN.

Step 405: The UE is synchronized to the target eNB. The UE accesses to atarget cell via a Random Access Channel (RACH). The UE stopstransmitting uplink data at the source eNB.

In order to solve the problem in the uplink data transmission in thepresent invention, a third method is as follows: according to the uplinkreceive status received in the message of the step 404 and incombination with the uplink data that is forwarded by the source eNB andreceived from the source eNB, the target eNB updates the uplink receivestatus and then transmits the updated uplink receive status to the UE.If the target eNB still continues receiving data forwarded from thesource eNB after the RACH is successful, the target eNB forms a newuplink receive status upon receiving all forwarded data from the sourceeNB. The target eNB knows that the source eNB has finished dataforwarding according to an end marker. Corresponding to the thirdmethod, steps 406 and 406 a may not be executed. Corresponding to thethird method, steps 408 and 408 a may also not be executed.

Step 406: By the target eNB, an uplink data receipt stopping indicationis transmitted to the source eNB. The message for transmitting theuplink data receipt stopping indication contains a new eNB UE X2AP IDand an old eNB UE X2AP ID of the UE. Upon receiving the message, thesource eNB stops receiving the uplink data from the UE.

The method provided by the present invention is described by taking anX2 handover process as example. If the handover is an S1 handover, thetarget eNB transmits the uplink data receipt stopping indication to thesource eNB by a Mobility Management Entity (MME). The S1 message fortransmitting the uplink data receipt stopping indication contains an eNBeNB UE S1AP ID and an MME UE S1AP ID.

In the method provided by the present invention, this step is anoptional and may or may not be executed. When this step is not executed,step 406 a is also not to be executed. When this step is not executed,upon receiving the message of step 408, the source eNB stops receivingthe uplink data from the UE. Through this process, the source eNB cantimely know that the UE stops transmitting the data to the source eNB,so that the source eNB does not need an idle air interface for datamonitoring.

Step 406 a: By the source eNB, a third SN status transfer message istransmitted to the target eNB. The message contains a receive status ofan uplink PDCP SN and/or HFN, and this message also contains a receivestatus of an uplink PDCP data packets and an uplink count. The third SNstatus transfer message and the SN status transfer in the step 404 canbe a same message or different messages. When the third SN statustransfer message is transmitted, it is indicated that the source eNB hasfrozen the receive status. In the method provided by the presentinvention, this step is an optional and may or may not be executed.

Step 407: By the UE, an RRC connection reconfiguration completionmessage is transmitted to the target eNB. After the step 403, the sourceeNB simultaneously transmits data to the UE and forwards data to thetarget eNB. Therefore, a part of the forwarded data received from thesource eNB by the target eNB may be already received by the UE.According to a PDCP status report received from the UE, the target eNBdetects duplicated data that is not to be transmitted to the UE, thendiscards the duplicated data and directly transmits data that has beennot received by the UE. The target eNB transmits data packets to the UE,starting from a data packet having the next PDCP SN expected by the UEin the PDCP status report. The UE stops receiving the downlink data fromthe source eNB.

The target eNB knows the next data packet that is expected to bereceived according to the receive status of uplink PDCP data packetsreceived in the step 406 a, and the target eNB transmits thisinformation to the UE. Thus, the UE, transmits, to the target eNB, thedata packet staring from the next data packet that has not been receivedfrom the network side. The target eNB knows the next data packet to betransmitted to the SGW according to the uplink count received from thesource eNB in the step 406 a, so that a data packet is from beingrepeatedly transmitted to the SGW or being missed.

According to the uplink receive status received from the target eNB andin combination with the data transmitted by the UE at the source eNB andthe feedback received from the source eNB, the UE transmits uplink datapackets to the target eNB starting from the next data packet that hasnot been received by the source eNB.

Step 408: By the target eNB, a data transmission stopping indication istransmitted to the source eNB. The message for transmitting the datatransmission stopping indication contains a new eNB UE X2AP ID and anold eNB UE X2AP ID of the UE. Upon receiving this message, the sourceeNB stops transmitting the downlink data to the UE. If the step 406 isnot executed, the source eNB also stops receiving the uplink data fromthe UE.

The data forwarded to the target eNB by the source eNB can contain PDCPdata packets containing an SN, and PDCP data packets containing no SN.The source eNB informs the target eNB of the next PDCP SN to be used viastep 408 a.

Through this process, the source eNB can timely know that the UE stopsreceiving the data from the source eNB, so that the source eNB does notneed an idle air interface for downlink data transmission.

There are following three ways for the target eNB to detect data that isnot to be transmitted to the UE.

Way 1: According to a transmission status of a PDCP SN and/or HFNreceived from the source eNB in the step 408 a, the target eNB discardsduplicated data and directly transmits data that has been not receivedby the UE. The duplicated data refers to data that is transmitted to theUE and forwarded to the target eNB simultaneously by the source eNB.

Way 2: According to a PDCP status report received from the UE, thetarget eNB detects duplicated data that is not to be transmitted to theUE, then discards the duplicated data and directly transmits data thathas been not received by the UE. The target eNB transmits data packetsto the UE, starting from a data packet having the next PDCP SN expectedby the UE in the PDCP status report.

Way 3: In combination with the transmission status of PDCP SN and/or HFNreceived in the step 408 a and the PDCP status report received from theUE, the target eNB detects duplicated data that is not to be transmittedto the UE, then discards the duplicated data and directly transmits datathat has been not received by the UE. In other words, in combinationwith the second SN status transfer message and the PDCP status reportreceived from the UE, the target eNB detects duplicated data that is notto be transmitted to the UE, then discards the duplicated data anddirectly transmits data that has been not received by the UE, the secondSN status transfer message containing the transmission status of thePDCP SN and/or HFN, where the second SN status transfer message containsthe transmission status of the PDCP SN and/or HFN.

The target eNB knows the next data packet that is expected to bereceived according to the receive status of uplink PDCP data packetsreceived in the step 406 a, and the target eNB transmits thisinformation to the UE. Thus, at the target eNB, the UE transmitsstarting from the next data packet that has not been received from thenetwork side. The target eNB knows the next data packet to betransmitted to the SGW according to the uplink count received from thesource eNB in the step 406 a, so that a data packet is from beingrepeatedly transmitted to the SGW or being missed.

The method provided by the present invention is described by taking anX2 handover process as example. If the handover is an S1 handover, thetarget eNB transmits the data transmission stopping indication to thesource eNB via a Mobility Management Entity (MME). The S1 message fortransmitting the data transmission stopping indication contains an eNBeNB UE S1AP ID and an MME UE S1AP ID.

Step 408 a: By the source eNB, a fourth SN status transfer message istransmitted to the target eNB. The message contains a transmissionstatus of a downlink PDCP SN and/or HFN, and the message also contains adownlink count. The fourth SN status transfer message and the SN statustransfer in the step 404 can be a same message or different messages.When the fourth SN status transfer message is transmitted, it isindicated that the source eNB has frozen the transmission status. If thesteps 406 and 406 a are not executed, the message further contains areceive status of the uplink PDCP SDU and an uplink count. When thefourth status transfer message is transmitted, it is also indicated thatthe source eNB has frozen the receive status.

In the method provided by the present invention, this step is anoptional and may or may not be executed.

Step 409: By the target eNB, a path handover request message istransmitted to an MME.

Step 410: By the MME, a path handover request response message istransmitted to the target eNB.

Step 411: By the target eNB, a UE context release message is transmittedto the source eNB.

Now, the second method for supporting seamless handover provided by thepresent invention has been described. By this method, the unnecessarydata transmission to the UE from the source eNB and unnecessary uplinkdata channel monitoring of the source eNB can be avoided. Actually, theUE has stopped the downlink receipt and uplink transmission at thesource eNB, so the air interface resources and power loss are saved. Bythis method, the continuous transmission of uplink and downlink data canbe ensured, and the missing or duplication transmission of data can beavoided.

FIG. 5 shows a third method for supporting seamless handover accordingto the present invention. This method comprises the following steps.

Step 501: By a source eNB, a handover request message is transmitted toa target eNB.

Step 502: By the target eNB, a handover request acknowledge message istransmitted to the source eNB.

Step 503: By the source eNB, an RRC connection reconfiguration messageis transmitted to a UE. The source eNB continues transmitting downlinkdata to the UE. The source eNB continues receiving uplink data from theUE. Upon receiving this message, the UE continues receiving the downlinkdata transmitted by the source eNB, and continues transmitting theuplink data to the source eNB.

In order to solve the problem in the uplink data transmission in thepresent invention, one corresponding method is that the source eNBcontinues feeding an uplink data receive status back to the UE afterthis step or after step 504. Thus, the UE knows which data packets havebeen already received from the UE by the source eNB. Accordingly, upontransmitting a message in step 507, the UE knows a data packet startingfrom which the data packet should be transmitted to the target eNB.

In order to solve the problem in the uplink data transmission in thepresent invention, a second method is that the source eNB transmits anuplink receive status after stopping receiving the uplink data to thetarget eNB through step 508 a. Thus, the target eNB can accurately knowan uplink receive status of the source eNB after the UE stopstransmitting the uplink data, so that the target eNB can inform the UEof this, and the UE then transmits data packets that have not beenreceived by the source eNB to the target eNB. Corresponding to thismethod, more preferably, step 504 may be omitted and not executed. Thesource eNB transmits an uplink count after stopping receiving the uplinkdata to the target eNB via step 508 a. Thus, the target eNB knows a PDCPpacket from starting which the data is transmitted to an SGW, accordingto the received uplink count. Corresponding to this method, morepreferably, after the step 504 and before the step 508 or 508 a, thesource eNB can directly transmit the sequentially received uplink datapackets to the SGW, and forward other data packets to the target eNB.

Step 504: By the source eNB, a Sequence Number (SN) status transfer istransmitted to the target eNB.

In this step, the source eNB does not freeze the transmission andreceive status.

Step 505: The source eNB starts to forward data to the target eNB.

In order to solve the problem in the missing or duplication transmissionof downlink data in an enhanced handover process, there are thefollowing several methods.

Method 1: An order of forwarded data packets is identified by an SN in aGTP-U header. The target eNB knows a sequential order of receiving PDCPdata packets according to the SN in the GTP-U header, then identifiesthe first data packet without a PDCP SN according to the SN to be usedby the next PDCP received from the source eNB, and successivelyidentifies subsequent data packets. Thus, upon receiving the next PDCPSN to be transmitted from the UE, the target eNB can correctly transmitthe next PDCP data packet that has not been received by the UE. Even ifthe X2 interface cannot realize sequential transmission, according tothe SN in the GTP-U header, the target eNB still knows the correct orderof the data packets transmitted by the source eNB, so that the correctPDCP data packet is marked with the next PDCP SN, thereby avoiding themissing or duplication transmission of the data packets.

Method 2: The process of transmitting an SN status transfer message tothe target eNB by the source eNB in the step 504 is not executed. Thesource eNB contains an SN in all PDCP data packets forwarded to thetarget eNB, and the data packets carrying the SN are also to betransmitted to the UE. In step 508 a, the source eNB informs the targeteNB of the PDCP uplink receive status and uplink and downlink count ofthe source eNB. Thus, the target eNB knows a downlink transmissionstatus on the source side when the source eNB stops transmitting thedownlink data, and an uplink receive status when the source eNB stopsreceiving the uplink data packets, and a transmission status of theuplink data transmitted by the source eNB to the SGW. Meanwhile, theproblem in the uplink and downlink data transmission in the presentinvention is solved, and no missing and duplication transmission of datais ensured.

Method 3: A FP is defined in the data packets forwarded to the targeteNB by the source eNB. A data portion of the FP contains the forwardedPDCP data packets, and a packet header of the FP contains a sequencenumber of the forwarded data packets. The target eNB knows a sequentialorder of receiving PDCP data packets according to the SN in the FPheader, then identifies the first data packet without the PDCP SNaccording to the SN to be used by the next PDCP received from the sourceeNB, and successively identifies subsequent data packets. Thus, uponreceiving the next PDCP SN to be transmitted from the UE, the target eNBcan correctly transmit the next PDCP data packet that has not beenreceived by the UE. Even if the X2 interface cannot realize sequentialtransmission, according to the SN in the FP header, the target eNB stillknows the correct order of the data packets transmitted by the sourceeNB, so that the correct PDCP data packet is marked with the next PDCPSN, thereby avoiding the missing or duplication transmission of the datapackets.

Method 4: All data packets forwarded to the target eNB by the source eNBcontain a PDCP SN. The next PDCP SN to be used, contained in the datapackets transmitted to the target eNB by the source eNB, is dummy. Uponreceiving the forwarded data packets, the target eNB ignores thereceived next PDCP SN to be used if it is found that all the forwardeddata packets contain the SN. Or, the next PDCP SN to be used containedin the data packets transmitted to the target eNB by the source eNB is aparticular value. Upon receiving the SN status transfer message, thetarget eNB ignores the received next PDCP SN to be used if it is foundthat the next PDCP SN to be used is a particular value.

Method 5: The source eNB estimates the number of PDCP data packetstransmitted to the UE after receiving the SN status transfer message.Upon transmitting a corresponding number of data packets, the source eNBstops transmitting data to the UE. When forwarding a correspondingnumber of data packets to the target eNB, the source eNB contains a PDCPSN in the data packets, but does not contain a PDCP SN in the subsequentdata packets. The target eNB uses the received next PDCP SN to be usedto represent the received data packet without a PDCP SN.

Step 506: The UE is synchronized to the target eNB. The UE accesses to atarget cell via a Random Access Channel (RACH).

In order to solve the problem in the uplink data transmission in thepresent invention, a third method is as follows: according to the uplinkreceive status received in the message of the step 504 and incombination with the uplink data that is forwarded by the source eNB andreceived from the source eNB, the target eNB updates the uplink receivestatus and then transmits the updated uplink receive status to the UE.If the target eNB still continues receiving data forwarded from thesource eNB after the RACH is successful, the target eNB forms a newuplink receive status upon receiving all forwarded data from the sourceeNB. The target eNB knows that the source eNB has finished dataforwarding according to an end marker. Corresponding to the thirdmethod, steps 508 and 508 a may not be executed.

Step 507: By the UE, an RRC connection reconfiguration completionmessage is transmitted to the target eNB. After the step 403, the sourceeNB simultaneously transmits data to the UE and forwards data to thetarget eNB. Therefore, a part of the forwarded data received from thesource eNB by the target eNB may be already received by the UE.According to a PDCP status report received from the UE, the target eNBdetects duplicated data that is not to be transmitted to the UE, thendiscards the duplicated data and directly transmits data that has beennot received by the UE. The target eNB transmits data packets to the UE,starting from a data packet having the next PDCP SN expected by the UEin the PDCP status report. The UE stops receiving the downlink data fromthe source eNB, and stops transmitting the uplink data to the sourceeNB.

According to the uplink receive status received from the target eNB andin combination with the data transmitted by the UE at the source eNB andthe feedback received from the source eNB, the UE transmits uplink datapackets to the target eNB starting from the next data packet that hasnot been received by the source eNB.

Step 508: By the target eNB, a data transmission stopping indication istransmitted to the source eNB. The message for transmitting the datatransmission stopping indication contains a new eNB UE X2AP ID and anold eNB UE X2AP ID of the UE. Upon receiving this message, the sourceeNB stops transmitting the downlink data to the UE. The source eNB stopsreceiving the uplink data from the UE.

The method provided by the present invention is described by taking anX2 handover process as example. If the handover is an S1 handover, thetarget eNB transmits the data transmission stopping indication to thesource eNB by a Mobility Management Entity (MME). The S1 message fortransmitting the data transmission stopping indication contains an eNBeNB UE S1AP ID and an MME UE S1AP ID.

The data forwarded to the target eNB by the source eNB can contain PDCPdata packets containing an SN, and PDCP data packets containing no SN.The source eNB informs the target eNB of the next PDCP SN to be used viastep 508 a.

Through this process, the source eNB can timely know that the UE stopsreceiving and transmitting the data at the source eNB, so that thesource eNB does not need an idle air interface for data transmission oruplink receipt.

Step 508 a: By the source eNB, a fifth SN status transfer message istransmitted to the target eNB. The message contains a receive status ofan uplink PDCP SN and/or HFN. The message contains a transmission statusof a downlink PDCP SN and/or HFN. The message contains a receive statusof an uplink PDCP SDU, an uplink count and a downlink count. The fifthSN status transfer message and the SN status transfer in the step 504can be a same message or different messages. When the second statustransfer message is transmitted, it is indicated that the source eNB hasfrozen the transmission and receive status.

The target eNB knows the next data packet that is expected to bereceived according to the receive status of uplink PDCP data packetsreceived in the step 508 a, and the target eNB transmits thisinformation to the UE. Thus, the UE transmits, to the target eNB, thedata packet starting from the next data packet that has not beenreceived from the network side. The target eNB knows the next datapacket to be transmitted to the SGW according to the uplink countreceived from the source eNB in the step 508 a, so that a data packet isfrom being repeatedly transmitted to the SGW or being missed.

Step 509: By the target eNB, a path handover request message istransmitted to an MME.

Step 510: By the MME, a path handover request response message istransmitted to the target eNB.

Step 511: By the target eNB, a UE context release message is transmittedto the source eNB.

Now, the third method for supporting seamless handover according to thepresent invention has been described. By this method, the unnecessarydata transmission to the UE by the source eNB and the unnecessarymonitoring of the uplink data channel can be avoided. However, actually,the UE has stopped the downlink receipt and uplink transmission at thesource eNB, so the air interface resources and power loss are saved.

FIG. 6 is a schematic diagram of an embodiment for dual connectivity ofthe first method for supporting seamless handover according to thepresent invention. This method comprises the following steps.

Step 601: By a Master eNB (MeNB), a Secondary eNB (SeNB) additionrequest message is transmitted to a Target SeNB (T-SeNB).

Step 602: By the T-SeNB, an SeNB addition request acknowledge message istransmitted to the MeNB.

Step 603: If resources for the T-SeNB are allocated successfully, by theMeNB, an SeNB release request message is transmitted to a Source SeNB(S-SeNB). If the data is to be forwarded, the MeNB provides the S-SeNBwith a data forwarding address. Upon receiving the SeNB release message,the S-SeNB continues transmitting the data to a UE. The S-SeNB can startto forward the data.

Corresponding to a split bearer, the S-SeNB continues transmittinguplink data to the MeNB. Corresponding to a Second Cell Group (SCG)bearer, the S-SeNB continues transmitting uplink data to an SGW.

For the SCG bearer, the S-SeNB continues feeding an uplink data receivestatus to the UE after this step. Thus, the UE knows which data packetshave been already received from the UE by the S-SeNB. Accordingly, upontransmitting the message in the step 608, the UE knows a data packetstarting from which the data packet should be transmitted to the targeteNB.

In another method provided by the present invention, this step can beomitted and not be executed, so that the S-SeNB will continuetransmitting data to the UE.

Step 604: By the S-SeNB, the data is forwarded to the T-SeNB.Corresponding to the split bearer, the S-SeNB forwards the data to theMeNB, and then the MeNB forwards the data to the T-SeNB. For the SCGbearer, like the split bearer, the S-SeNB can forward the data to theT-SeNB via the MeNB, or the S-SeNB can directly forward the data to theT-SeNB. Depending on the implementation, the forwarding the data to theT-SeNB by the S-SeNB can be executed at any time after the step 603.

Step 605: By the MeNB, an RRC connection reconfiguration message istransmitted to a UE. The MeNB continues transmitting downlink data tothe S-SeNB. Meanwhile, the MeNB also forwards the downlink data to theT-SeNB.

Step 606: The RRC connection reconfiguration message is received by theUE. The UE does not delete or reset the configuration for the S-SeNB.The UE continues receiving the downlink data transmitted by the S-SeNB.The UE continues transmitting the uplink data to the S-SeNB. The UEtransmits an RRC connection reconfiguration completion message to theMeNB.

Step 607: By the MeNB, an SeNB reconfiguration completion message istransmitted to the T-SeNB.

Step 608: The UE is synchronized to the T-SeNB.

The UE stops receiving the downlink data from the S-SeNB, and also stopstransmitting the uplink data at the S-SeNB.

Step 609: By the T-SeNB, a data transmission stopping indication istransmitted to the MeNB. The message for transmitting the datatransmission stopping indication contains an eNB UE X2AP ID allocatedfor the UE by the T-SeNB and an eNB UE X2AP ID allocated for the MeNB bythe T-SeNB. Upon receiving this message, the MeNB stops transmitting thedownlink data to the S-SeNB.

Step 610: By the MeNB, the data transmission stopping indication istransmitted to the S-SeNB. The message for transmitting the datatransmission stopping indication contains an eNB UE X2AP ID allocatedfor the UE by the MeNB and an eNB UE X2AP ID allocated for the S-SeNB bythe MeNB. Upon receiving this message, the S-SeNB stops transmitting thedownlink data to the UE. The S-SeNB stops receiving the uplink data fromthe UE.

Through the processes in the steps 609 and 610, the S-SeNB can timelyknow that the UE stops receiving and transmitting the data at theS-SeNB, so that the S-SeNB does not need an idle air interface for datatransmission or uplink receipt.

Step 611 a: By the S-SeNB, an SN status transfer is transmitted to theMeNB.

Step 611 b: By the MeNB, the SN status transfer is transmitted to theT-SeNB.

For the SCG bearer, the execution of the steps 611 a and 611 b after thestep 610 has the following advantages: the T-SeNB can accurately knowthe receive status of the S-SeNB after the UE stops transmitting theuplink data, so that the T-SeNB can inform the UE of this and the UEthen can transmit data packets that have not been received by the S-SeNBto the T-SeNB. Corresponding to this method, more preferably, after thestep 603 and before the step 610, the S-SeNB can directly transmit thesequentially received uplink data packets to the SGW, and forward otherdata packets to the T-SeNB.

For the SCG bearer, the T-SeNB knows the next data packet that isexpected to be received according to the receive status of uplink PDCPdata packets received in the message of the step 611 b, and the T-SeNBtransmits this information to the UE. Thus, the UE transmits, to theT-SeNB, the data packet starting from the next data packet that has notbeen received from the network side. The T-SeNB knows the next datapacket to be transmitted to the SGW according to the uplink countreceived from the S-SeNB in the step 611 b, so that a data packet isfrom being repeatedly transmitted to the SGW or being missed.

For the split bearer, the steps 611 a and 611 b can be executed at anytime after the step 603.

For the SCG bearer, the steps 611 a and 611 b can be executed after thestep 610.

The detailed descriptions of steps irrelevant to the main contents ofthe present application are omitted here, for example, the ERABmodification indication process between the MeNB and the MME.

Now, the embodiment for dual connectivity of the first method forsupporting seamless handover provided by the present application hasbeen described. By this method, the unnecessary data transmission to theUE by the S-SeNB or the unnecessary data monitoring of an uplink datachannel can be avoided. However, actually, the UE has stopped thedownlink receipt and uplink transmission at the S-SeNB, so the airinterface resources and power loss are saved. By this method, thecontinuous transmission of uplink and downlink data can be ensured, andthe missing or duplication transmission of data can be avoided.

FIG. 7 shows a fourth method for supporting seamless handover accordingto the present invention. This method comprises the following steps.

Step 701: By a source eNB, a handover request message is transmitted toa target eNB.

In this embodiment, the source eNB decides to adopt an enhanced handoverprocess. The enhanced handover refers a handover scheme for maintaininga connection to the source eNB, as discussed in the 3GPP Rel-14.Specifically, the enhanced handover process means that the source eNBstill transmits downlink data to a UE and also receives uplink data fromthe UE during the handover execution process. More specifically, afterthe source eNB has transmitted an RRC connection reconfigurationmessage, or after the source eNB has transmitted an RRC connectionreconfiguration message and before the target eNB receives an RACHaccess from the UE, or after the source eNB has transmitted an RRCconnection reconfiguration message and before the target eNB receivesthe RRC connection reconfiguration from the UE, the source eNB stilltransmits downlink data to the UE and also receives uplink data from theUE. The meaning of the handover execution process is the same as thedefinition in the 3GPP TS36.300. The source eNB decides to adopt theenhanced handover process according to its support for the enhancedhandover process and the capability of the UE. Or, the source eNBdecides to adopt the enhanced handover process according to its supportfor the enhanced handover process, the capability of the UE, and thecapability for supporting the enhanced handover process of the targeteNB. The source eNB can also decide to adopt the enhanced handoverprocess by taking the requirements of a service (e.g., Quality ofService (QoS) information) into consideration. The source eNB can alsodecide to adopt the enhanced handover process by taking otherinformation into consideration, without influencing the main contents ofthe present invention. The source eNB acquires, from the UE, thecapability for supporting the enhanced handover of the UE. Thecapability for supporting the enhanced handover of the UE means that,upon receiving the RRC connection reconfiguration message, the UE maynot freeze the transmission and receive status at the source eNB and notreset or empty the information at the layer 2 (L2) of the source eNB,and may continue receiving or transmitting data from the source eNB. Thesource eNB knows the capability for supporting the enhanced handoverprocess of the target eNB through an Operation & Maintenance (O&M)configuration or an X2 establishment process. Corresponding to the amethod using the X2 establishment process, both an X2 establishmentrequest message and an X2 establishment response message contain thecapability for supporting the enhanced handover process of an eNBtransmitting the message. An eNB receiving the X2 establishment requestmessage or the X2 establishment response message maintains the receivedcapability for supporting the enhanced handover process of the oppositeeNB.

In the present invention, there are following methods to indicate to thetarget eNB that the source eNB adopts the enhanced handover process.

Method 1: By containing enhanced handover indication information in thehandover request message, the source eNB indicates to the target eNBthat this handover process is an enhanced handover process.

Method 2: The capability for supporting the enhanced handover of the UEis contained in the handover request message, and the target eNB knowsthat this handover process is an enhanced handover process according tothe capability for supporting the enhanced handover process of thesource eNB and the capability for supporting the enhanced handover ofthe UE. The source eNB acquires, from the UE, the capability forsupporting the enhanced handover of the UE. The target eNB knows thecapability for supporting the enhanced handover process of the sourceeNB through an Operation & Maintenance (O&M) configuration or an X2establishment process. Corresponding to the a method using the X2establishment process, both an X2 establishment request message and anX2 establishment response message contain the capability for supportingthe enhanced handover process of an eNB transmitting the message. An eNBreceiving the X2 establishment request message or the X2 establishmentresponse message maintains the received capability for supporting theenhanced handover process of the opposite eNB. Corresponding to thismethod, if it is assumed that the source eNB supports the enhancedhandover process, the source eNB will initiate an enhanced handoverprocess when initiating a handover to a UE supporting the enhancedhandover. Or, if it is assumed that both the source eNB and the targeteNB support the enhanced handover process, the source eNB will initiatean enhanced handover process when initiating a handover to a UEsupporting the enhanced handover.

The source eNB can inform the target eNB of the maximum number of datapackets transmitted to the UE starting from the first data packetforwarded to the target eNB during the enhanced handover process, forexample, the number of data packets within the maximum PDCP SN rangeminus 1, the number of data packets within the maximum PDCP SN range, ormore. For example, if the PDCP SN is from 0 to 127, the data packetswithin the maximum PDCP SN range minus 1 are 127 PDCP SDUs or PDUs.

Step 702: By the target eNB, a handover request acknowledge message istransmitted to the source eNB.

If the target eNB receives the enhanced handover indication informationfrom the source eNB, the target eNB contains the enhanced handoverindication information in an RRC container of the handover requestacknowledge message. The enhanced handover indication information istransmitted to the UE via the RRC container.

Step 703: By the source eNB, an RRC connection reconfiguration messageis transmitted to a UE. This message contains the enhanced handoverindication information. Corresponding to the enhanced handover process,the source eNB continues transmitting downlink data to the UE.Corresponding to a method provided by the present invention, the datapackets continuously transmitted to the UE by the source eNB are at mostthe number of data packets within the maximum PDCP SN range minus 1starting from the first data packet forwarded to the target eNB. Forexample, if the first data packet forwarded to the target eNB by thesource eNB has a PDCP SN of 5 and an HFN of 10, the data packetstransmitted to the UE by the source eNB at most have a PDCP SN of 3 andan HFN of 11.

The source eNB continues receiving uplink data from the UE. Uponreceiving this message, the UE continues receiving the downlink datatransmitted by the source eNB, and continues transmitting the uplinkdata at the source eNB. According to the enhanced handover indicationinformation contained in the received RRC connection reconfigurationmessage, the UE knows that this handover process is an enhanced handoverprocess, so that the UE continues transmitting and receiving data at thesource eNB.

In order to solve the problem in the uplink data transmission in thepresent invention, one corresponding method is that the source eNBcontinues feedback an uplink data receive status back to the UE afterthis step or after step 704. Thus, the UE knows which data packets havebeen already received from the UE by the source eNB. Accordingly, upontransmitting a message in step 706, the UE knows a data packet staringfrom which the data packet should be transmitted to the target eNB.

Step 704: By the source eNB, a Sequence Number (SN) status transfer istransmitted to the target eNB.

In this step, the source eNB does not freeze the transmission andreceive status.

The source eNB starts to forward data to the target eNB. The source eNBalso forwards, to the target eNB, the downlink data that is transmittedto the UE. The source eNB forwards, to the target eNB, the uplink datapackets received from the UE. One method is as follows: the source eNBforwards, to the target eNB, all the received uplink packets includingthe data packets received in-sequence or out-of-sequence after the SNstatus transfer message. Another method is as follows: the source eNBtransmit the received data packets in sequence to an SGW, and forwardsthe received data packets out of sequence to the target eNB.

Step 705: The UE is synchronized to the target eNB. The UE accesses to atarget cell via a Random Access Channel (RACH).

In the present invention, the UE can stop receiving the downlink datafrom the source eNB and stop transmitting the uplink data at the sourceeNB in this step. Or, the UE can stop receiving the downlink data fromthe source eNB and stop transmitting the uplink data at the source eNBin step 706.

In order to solve the problem in the uplink data transmission in thepresent invention, according to the uplink receive status and the uplinkcount received in the message of the step 704 and in combination withthe uplink data that is forwarded by the source eNB and received fromthe source eNB, the target eNB updates the uplink receive status andthen transmits the updated uplink receive status to the UE. If thetarget eNB still continues receiving the data forwarded from the sourceeNB after the RACH is successful, the target eNB forms a new uplinkreceive status upon receiving all forwarded data from the source eNB.The target eNB knows that the source eNB has finished data forwardingaccording to an end marker.

Step 706: By the UE, an RRC connection reconfiguration completionmessage is transmitted to the target eNB. After the step 703, the sourceeNB simultaneously transmits the downlink data to the UE and forwardsthe downlink data to the target eNB. Therefore, a part of the forwardeddata received from the source eNB by the target eNB may be alreadyreceived by the UE. There are following ways for the target eNB todetect data that don't need to be transmitted to the UE.

According to the PDCP status report received from the UE, the target eNBknows the next PDCP SN expected to be received by the UE. The target eNBdetects duplicated data packets that don't need to be transmitted to theUE, and discards the data that has been received by the UE, andtransmits data packet to the UE directly starting from the next datapacket expected by the UE.

The PDCP status report transmitted to the target eNB by the UE merelycontains the PDCP SN of the next data packet expected to be received,but does not contain an HFN corresponding to the PDCP SN.

By using the method described in the step 701, the target eNB can knowwhether this handover process is an enhanced handover process. For theenhanced handover process, the target eNB transmits data to the UEand/or transmits uplink data to the SGW by the method for the enhancedhandover process. The target eNB determines an HFN corresponding to thePDCP SN of the next data packet expected to be received by the UE,according to the enhanced handover process.

The target eNB considers that the data packet corresponding to the nextexpected PDCP SN contained in the PDCP status received from the UE is afirst data packet corresponding to this PDCP SN stored in a buffer ofthe target eNB. The maximum number of data packets transmitted to the UEby the source eNB starting from the first data packet forwarded to thetarget eNB is the number of data packets within the maximum PDCP SNrange minus 1. For example, if the first data packet received from thesource eNB by the target eNB has a PDCP SN of 5 and an HFN of 10, thedata packets transmitted to the UE by the source eNB at most have a PDCPSN of 3 and an HFN of 11. Therefore, the maximum data packet receivedfrom the source eNB by the UE has a PDCP SN of 3 and an HFN of 11. Ifthe data transmitted by the source eNB all has been received by the UE,the UE expects that the next received data packet has a PDCP SN of 4 andan HFN of 11. Therefore, this data packet is a first corresponding datapacket having a PDCP SN of 4 in the buffer of the target eNB, and theHFN corresponding to this data packet is 11. If some data packetstransmitted by the source eNB have not been received by the UE, forexample, if the next expected PDCP SN in the PDCP status received fromthe UE by the target eNB is 9, this data packet is a first correspondingdata packet having a PDCP SN of 9 in the buffer of the target eNB, andthe target eNB knows that the corresponding HFN is 10. Therefore, ifthere are data packets having a same PDCP SN in the buffer of the targeteNB, a first data packet not receiving a data packet SN in the firstcorresponding UE PDCP status report is the next data packet to betransmitted to the UE. In this way, the target eNB knows an HFNcorresponding to this data packet.

Or, the target eNB considers that the next expected PDCP SN contained inthe PDCP status received from the UE is not far from a half of the PDCPSN range of the PDCP SN in the DL count received in the SN statustransfer message. In other words, if the maximum value of the PDCP SN isN, the next expected PDCP SN is not far from N/2 of the PDCP SN in theDL count. The source eNB also follows this principle, when transmittingdata to the UE after transmitting the SN status transfer message. Forexample:

-   -   in the DL count, the HFN is 10, and the PDCP SN is 8;    -   the PDCP SN is 7 bits (the range of the SN is from 0 to 127);    -   when the next expected PDCP SN received from the UE by the        target eNB is 73 to 127, the HFN is 9; and    -   when the next expected PDCP SN received from the UE by the        target eNB is 0 to 72, the HFN is 10.

Upon transmitting the SN status transfer message, the source eNBcontinues transmitting the data to the UE, and then stops transmittingdownlink data packets to the UE after the PDCP SN is 72 and the HFN is10.

Corresponding to the enhanced handover process, if the next expectedPDCP SN in the PDCP status received from the UE by the target eNB is 9,the target eNB knows that the corresponding HFN is 10. If the handoveris not an enhanced handover, the target eNB knows that the correspondingHFN is 9.

In order to solve the problem in the uplink data transmission in thepresent invention, according to the uplink receive status and the uplinkcount received in the message of the step 704 and in combination withthe uplink data that is forwarded by the source eNB and received fromthe source eNB, the target eNB updates the uplink receive status andthen transmits the updated uplink receive status to the UE. The targeteNB needs to form a new uplink receive status after all the forwardeddata has been received from the source eNB. The target eNB knows thatthe source eNB has finished data forwarding according to an end marker.The target eNB transmits the generated uplink receive status to the UE.

According to the uplink receive status received from the target eNB andin combination with the data transmitted by the UE at the source eNB andthe feedback received from the source eNB, the UE transmits uplink datato the target eNB starting from the next data packet that has not beenreceived by the source eNB.

Step 707: By the target eNB, a path handover request message istransmitted to an MME.

Step 708: By the MME, a path handover request response message istransmitted to the target eNB.

Step 709: By the target eNB, a UE context release message is transmittedto the source eNB.

Now, the fourth method for supporting seamless handover provided by thepresent invention has been described. By this method, the interruptiontime of data transmission during the handover process can be reduced,the continuous transmission of uplink and downlink data can be ensured,and the missing or duplication transmission of data can be avoided.Particularly, the problem that the target eNB does not know the HFNcorresponding to the next PDCP SN expected by the UE is solved. Thus,even if the source eNB and the target eNB are from differentmanufacturers, the interoperability between the two eNBs is ensured, andthe interruption time of data transmission during the handover processis reduced while ensuring the operability during the enhanced handoverprocess.

FIG. 8 shows a fifth method for supporting seamless handover accordingto the present invention. This method comprises the following steps.

Step 801: By a source eNB, a handover request message is transmitted toa target eNB.

Step 802: By the target eNB, a handover request acknowledge message istransmitted to the source eNB.

Step 803: By the source eNB, an RRC connection reconfiguration messageis transmitted to a UE. Corresponding to the enhanced handover process,the source eNB continues transmitting downlink data to the UE. Thesource eNB continues receiving uplink data from the UE. Upon receivingthis message, the UE continues receiving the downlink data transmittedby the source eNB, and continues transmitting the uplink data at thesource eNB.

In order to solve the problem in the uplink data transmission in thepresent invention, one corresponding method is that the source eNBcontinues feeding an uplink data receive status back to the UE afterthis step or after step 804. Thus, the UE knows which data packets havebeen already received from the UE by the source eNB. Accordingly, upontransmitting a message in step 806, the UE knows a data packet staringfrom which the data packet should be transmitted to the target eNB.

Step 804: By the source eNB, a Sequence Number (SN) status transfer istransmitted to the target eNB.

In this step, the source eNB does not freeze the transmission andreceive status.

The source eNB starts to forward data to the target eNB. The source eNBalso forwards, to the target eNB, the downlink data that is transmittedto the UE. The source eNB forwards, to the target eNB, the uplink datapackets received from the UE. One method is as follows: the source eNBforwards, to the target eNB, all the received uplink packets includingthe data packets received in-sequence or out-of-sequence after the SNstatus transfer message. Another method is as follows: the source eNBtransmit the received data packets in sequence to an SGW, and forwardsthe received data packets out of sequence to the target eNB.

Step 805: The UE is synchronized to the target eNB. The UE accesses to atarget cell via a Random Access Channel (RACH).

In the present invention, the UE can stop receiving the downlink datafrom the source eNB and stop transmitting the uplink data at the sourceeNB in this step. Or, the UE can stop receiving the downlink data fromthe source eNB and stop transmitting the uplink data at the source eNBin step 806.

In order to solve the problem in the uplink data transmission in thepresent invention, according to the uplink receive status and the uplinkcount received in the message of the step 804 and in combination withthe uplink data that is forwarded by the source eNB and received fromthe source eNB, the target eNB updates the uplink receive status andthen transmits the updated uplink receive status to the UE. If thetarget eNB still continues receiving the data forwarded from the sourceeNB after the RACH is successful, the target eNB forms a new uplinkreceive status upon receiving all forwarded data from the source eNB.The target eNB knows that the source eNB has finished data forwardingaccording to an end marker.

Step 806: By the UE, an RRC connection reconfiguration completionmessage is transmitted to the target eNB. After the step 803, the sourceeNB simultaneously transmits the downlink data to the UE and forwardsthe downlink data to the target eNB. Therefore, a part of the forwardeddata received from the source eNB by the target eNB may be alreadyreceived by the UE. There are following ways for the target eNB todetect data that don't need to be transmitted to the UE.

According to the PDCP status report received from the UE, the target eNBknows the next PDCP SN expected to be received by the UE. The target eNBdetects duplicated data packets that do not need to be transmitted tothe UE, and discards the data that has been received by the UE, andtransmits data packet to the UE directly starting from the next datapacket expected by the UE.

The PDCP status report transmitted to the target eNB by the UE containsthe PDCP SN of the next data packet expected to be received and an HFNcorresponding to the PDCP SN. The UE can merely contain an HFNcorresponding to the PDCP SN of the next data packet expected to bereceived in the PDCP status transmitted to an eNB during the enhancedhandover process. If the handover is not an enhanced handover, thetarget eNB knows the HFN corresponding to the PDCP SN of the next datapacket expected to be received according to the existing principle.

In order to solve the problem in the uplink data transmission in thepresent invention, according to the uplink receive status and the uplinkcount received in the message of the step 704 and in combination withthe uplink data that is forwarded by the source eNB and received fromthe source eNB, the target eNB updates the uplink receive status andthen transmits the updated uplink receive status to the UE. The targeteNB needs to form a new uplink receive status after all the forwardeddata has been received from the source eNB. The target eNB knows thatthe source eNB has finished data forwarding according to an end marker.The target eNB transmits the generated uplink receive status to the UE.

According to the uplink receive status received from the target eNB andin combination with the data transmitted by the UE at the source eNB andthe feedback received from the source eNB, the UE transmits uplink datato the target eNB starting from the next data packet that has not beenreceived by the source eNB.

Step 807: By the target eNB, a path handover request message istransmitted to an MME.

Step 808: By the MME, a path handover request response message istransmitted to the target eNB.

Step 809: By the target eNB, a UE context release message is transmittedto the source eNB.

Now, the fifth method for supporting seamless handover provided by thepresent invention has been described. By this method, the interruptiontime of data transmission during the handover process can be reduced,the continuous transmission of uplink and downlink data can be ensured,and the missing or duplication transmission of data can be avoided.Particularly, the problem that the target eNB does not know an HFNcorresponding to the next PDCP SN expected by the UE is solved. Thus,even if the source eNB and the target eNB are from differentmanufacturers, the interoperability between the two eNBs is ensured, andthe interruption time of data transmission during the handover processis reduced while ensuring the operability during the enhanced handoverprocess.

FIG. 9 is a schematic diagram of an embodiment for dual connectivity ofthe fourth and fifth methods for supporting seamless handover accordingto the present invention. This method comprises the following steps.

Step 901: By a Master eNB (MeNB), a Secondary eNB (SeNB) additionrequest message is transmitted to a Target SeNB (T-SeNB).

The MeNB decides to adopt an enhanced handover process. The principlefor the enhanced handover process is the same as that in the step 701.Specifically, in the SeNB change process, the enhanced handover processmeans that the S-SeNB still transmits downlink data to a UE and stillreceives uplink data from the UE during the handover execution process.More specifically, after the MeNB has transmitted an RRC connectionreconfiguration message, or after the MeNB has transmitted an RRCconnection reconfiguration message and before the T-SeNB receives anRACH access from the UE, or after the MeNB has transmitted an RRCconnection reconfiguration message and before the MeNB receives the RRCconnection reconfiguration from the UE, the S-SeNB still transmitsdownlink data to the UE and also receives uplink data from the UE. Inthis embodiment, the supporting the enhanced handover means supportingan enhanced SeNB change process.

The MeNB decides, according to the capability of the UE, the capabilityfor supporting the enhanced handover of the MeNB, and the capability forsupporting the enhanced handover of the S-SeNB and/or the T-SeNB,whether to adopt the enhanced handover. The MeNB can also decide toadopt the enhanced handover process by taking other information (e.g.,QoS) into consideration, without influencing the main contents of thepresent invention.

In the present invention, there are following methods to indicate theT-SeNB that an enhanced handover process is adopted.

Method 1: By containing enhanced handover indication information in theSeNB addition request message, the MeNB indicate to the T-SeNB that thishandover process is an enhanced handover process. The MeNB decides toadopt the enhanced handover process according to its support for theenhanced handover process and the capability of the UE. The source eNBcan also decide to adopt the enhanced handover process by consideringthe capability for supporting the enhanced handover process of theS-SeNB and/or the T-SeNB. The MeNB knows the capability for supportingthe enhanced handover process of the S-SeNB and/or the T-SeNB through anOperation & Maintenance (O&M) configuration or an X2 establishmentprocess. Corresponding to the a method using the X2 establishmentprocess, both an X2 establishment request and an X2 establishmentresponse message contain the capability for supporting the enhancedhandover process of an eNB transmitting the message. An eNB receivingthe X2 establishment request message or the X2 establishment responsemessage maintains the received capability for supporting the enhancedhandover process of the opposite eNB.

Method 2: The capability for supporting the enhanced handover of the UEis contained in the SeNB addition request message. If the T-SeNBsupports the enhanced handover process, the T-SeNB know that thishandover process is an enhanced handover process according to thecapability for supporting the enhanced handover process of the MeNBand/or the S-SeNB and the capability for supporting the enhancedhandover of the UE. The MeNB acquires, from the UE, the capability forsupporting the enhanced handover of the UE. The T-SeNB knows thecapability for supporting the enhanced handover process of the MeNBand/or the S-SeNB through an Operation & Maintenance (O&M) configurationor an X2 establishment process. Corresponding to the a method using theX2 establishment process, both an X2 establishment request message andan X2 establishment response message contain the capability forsupporting the enhanced handover process of an eNB transmitting themessage. An eNB receiving the X2 establishment request message or the X2establishment response message maintains the received capability forsupporting the enhanced handover process of the opposite eNB.Corresponding to this method, if it is assumed that the MeNB supportsthe enhanced handover process, the MeNB will initiate an enhancedhandover process when initiating a handover to a UE supporting theenhanced handover. Or, if it is assumed that all the MeNB, the S-SeNBand the T-SeNB support the enhanced handover process, the MeNB willinitiate an enhanced handover process when initiating a handover to a UEsupporting the enhanced handover.

Step 902: By the T-SeNB, an SeNB addition request acknowledge message istransmitted to the MeNB.

If the T-SeNB receives the enhanced handover indication information fromthe MeNB, the T-SeNB contains the enhanced handover indicationinformation in an RRC container of the SeNB addition request acknowledgemessage. The enhanced handover indication information is transmitted tothe UE via the RRC container. Here, the RRC container is a containerfrom the SeNB to the MeNB.

Step 903: By the MeNB, an SeNB release request message is transmitted toa Source SeNB (S-SeNB) if resources for the T-SeNB are allocatedsuccessfully. If the data is to be forwarded, the MeNB provides theS-SeNB with a data forwarding address.

In the present invention, there are following methods to indicate to theS-SeNB that an enhanced handover process is adopted by the MeNB.

Method 1: By containing enhanced handover indication information in theSeNB release request message, the MeNB indicates to the S-SeNB that thishandover process is an enhanced handover process. A method for decidingto adopt the enhanced handover process by the MeNB is the same as thatin the step 901 and will not be repeated here.

Method 2: The capability for supporting the enhanced handover of the UEis contained in the SeNB addition release message. If the S-SeNBsupports the enhanced handover process, the S-SeNB knows that thishandover process is an enhanced handover process according to thecapability for supporting the enhanced handover process of the MeNBand/or the T-SeNB and the capability for supporting the enhancedhandover of the UE. The MeNB acquires, from the UE, the capability forsupporting the enhanced handover of the UE. The S-SeNB knows thecapability for supporting the enhanced handover process of the MeNBand/or the T-SeNB through an Operation & Maintenance (O&M) configurationor an X2 establishment process. Corresponding to the a method using theX2 establishment process, both an X2 establishment request message andan X2 establishment response message contain the capability forsupporting the enhanced handover process of an eNB transmitting themessage. An eNB receiving the X2 establishment request message or the X2establishment response message maintains the received capability forsupporting the enhanced handover process of the opposite eNB.Corresponding to this method, if it is assumed that the MeNB supportsthe enhanced handover process, the MeNB will initiate an enhancedhandover process when initiating a handover process to a UE supportingthe enhanced handover. Or, if it is assumed that all the MeNB, theS-SeNB and the T-SeNB support the enhanced handover process, the MeNBwill initiate an enhanced handover process when initiating a handover toa UE supporting the enhanced handover.

Upon receiving the SeNB release message, the S-SeNB continuestransmitting data to the UE. The S-SeNB can start to forward the data.The SeNB continues receiving uplink data from the UE.

Corresponding to a split bearer, the S-SeNB continues transmittinguplink data to the MeNB. Corresponding to a Second Cell Group (SCG)bearer, the S-SeNB continues transmitting uplink data to an SGW.

For an SCG bearer, the S-SeNB continues feedback an uplink data receivestatus to the UE after this step. Thus, the UE knows which data packetshave been already received from the UE by the S-SeNB. Accordingly, upontransmitting a message in step 907, the UE knows a data packet staringfrom which the data packet should be transmitted to the T-SeNB.

Step 904: By the MeNB, an RRC connection reconfiguration message istransmitted to a UE. This message contains the enhanced handoverindication information. The MeNB continues transmitting downlink data tothe S-SeNB. The MeNB also forwards the downlink data to the T-SeNB.Corresponding to a method provided by the present invention, the datapackets continuously transmitted to the UE by the S-SeNB are at most thenumber of data packets within the maximum PDCP SN range minus 1 startingfrom the first data packet forwarded to the T-SeNB. For example, if thefirst data packet forwarded to the T-SeNB by the S-SeNB has a PDCP SN of5 and an HFN of 10, the data packets transmitted to the UE by the S-SeNBat most have a PDCP SN of 3 and an HFN of 11.

The UE receives the RRC connection reconfiguration message. The UE doesnot delete or reset the configuration for the S-SeNB. The UE continuesreceiving the downlink data transmitted by the S-SeNB. The UE continuestransmitting the uplink data to the S-SeNB. According to the enhancedhandover indication information contained in the received RRC connectionreconfiguration message, the UE knows that this handover process is anenhanced handover process, so that the UE continues transmitting andreceiving data at the source eNB.

Step 905: By the UE, an RRC connection reconfiguration completionmessage is transmitted to the MeNB.

Step 906: By the MeNB, an SeNB reconfiguration completion message istransmitted to the T-SeNB.

Step 907: The UE is synchronized to the T-SeNB.

The UE stops receiving the downlink data from the S-SeNB, and also stopstransmitting the uplink data to the S-SeNB.

Step 908 a: By the S-SeNB, an SN status transfer is transmitted to theMeNB.

Step 908 b: By the MeNB, the SN status transfer is transmitted to theT-SeNB.

Step 909: By the S-SeNB, the data is forwarded to the T-SeNB.Corresponding to the split bearer, the S-SeNB forwards the data to theMeNB, and then the MeNB forwards the data to the T-SeNB. For the SCGbearer, like the split bearer, the S-SeNB can forward the data to theT-SeNB via the MeNB, or the S-SeNB can directly forward the data to theT-SeNB. Depending on the implementation, the forwarding the data to theT-SeNB by the S-SeNB can be executed at any time after the step 903.

After the step 903, the S-SeNB simultaneously transmits the downlinkdata to the UE and forwards the downlink data to the T-SeNB. Therefore,a part of the forwarded data received from the S-SeNB by the T-SeNB maybe already received by the UE. The T-SeNB detects data that don't needto be transmitted to the UE in the following ways.

For the SCG bearer, according to the PDCP status report received fromthe UE, the T-SeNB knows the next PDCP SN expected to be received by theUE. The target eNB detects duplicated data packets that don't need to betransmitted to the UE, and discards the data that has been received bythe UE, and transmits data packet to the UE directly starting from thenext data packet expected by the UE.

For the SCG bearer, the PDCP status report transmitted to the T-SeNB bythe UE merely contains the PDCP SN of the next data packet expected tobe received, but does not contain an HFN corresponding to the PDCP SN.There are following two ways for the T-SeNB to know an HFN correspondingto the PDCP SN of the next data packet expected to be received.

Way 1:

By the method described in the step 901, the T-SeNB can know whetherthis handover process is an enhanced handover process. For the enhancedhandover process, the T-SeNB transmits data to the UE and/or transmitsuplink data to the SGW by the method for the enhanced handover process.

The T-SeNB considers that the data packet corresponding to the nextexpected PDCP SN contained in the PDCP status received from the UE is afirst data packet corresponding to this PDCP SN stored in a buffer ofthe T-SeNB. The maximum number of data packets transmitted to the UE bythe S-SeNB starting from the first data packet forwarded to the T-SeNBis the number of data packets within the maximum PDCP SN range minus 1.For example, if the first data packet received from the S-SeNB by theT-SeNB has a PDCP SN of 5 and an HFN of 10, the data packets transmittedto the UE by the S-SeNB at most have a PDCP SN of 3 and an HFN of 11.Therefore, the maximum data packet received from the S-SeNB by the UEhas a PDCP SN of 3 and an HFN of 11. If the data transmitted by theS-SeNB all has been received by the UE, the UE expects that the nextreceived data packet has a PDCP SN of 4 and an HFN of 11. Therefore,this data packet is a first corresponding data packet to having a PDCPSN of 4 in the buffer of the T-SeNB, and the HFN corresponding to thisdata packet is 11. If some data packets transmitted by the S-SeNB havenot been received by the UE, for example, if the next expected PDCP SNin the PDCP status received from the UE by the T-SeNB is 9, this datapacket is a first corresponding data packet having a PDCP SN of 9 in thebuffer of the T-SeNB, and the T-SeNB knows that the corresponding HFN is10. Therefore, if there are data packets having a same PDCP SN in thebuffer of the T-SeNB, a first data packet not receiving a data packet SNin the first corresponding UE PDCP status report is the next data packetto be transmitted to the UE. In this way, the T-SeNB knows an HFNcorresponding to this data packet.

Or, for the enhanced handover process, the T-SeNB considers that thenext expected PDCP SN contained in the PDCP status received from the UEis not far from a half of the PDCP SN range of the PDCP SN in the DLcount received in the SN status transfer message. In other words, if themaximum value of the PDCP SN is N, the next expected PDCP SN is not farfrom N/2 of the PDCP SN in the DL count. The S-SeNB also follows thisprinciple, when transmitting data to the UE after transmitting the SNstatus transfer message. For example:

-   -   in the DL count, the HFN is 10, and the PDCP SN is 8;    -   the PDCP SN is 7 bits (the range of the SN is from 0 to 127);    -   when the next expected PDCP SN received from the UE by the        T-SeNB is 73 to 127, the HFN is 9; and    -   when the next expected PDCP SN received from the UE by the        T-SeNB is 0 to 72, the HFN is 10.

Upon transmitting the SN status transfer message, the S-SeNB continuestransmitting the data to the UE, and then stops transmitting downlinkdata packets to the UE after the PDCP SN is 72 and the HFN is 10.

Corresponding to the enhanced handover process, if the next expectedPDCP SN in the PDCP status received from the UE by the T-SeNB is 9, theT-SeNB knows that the corresponding HFN is 10. If the handover is not anenhanced handover, the T-SeNB knows that the corresponding HFN is 9.

Way 2:

The PDCP status report transmitted to the T-SeNB by the UE contains thePDCP SN of the next data packet expected to be received and an HFNcorresponding to the PDCP SN. The UE can merely contain an HFNcorresponding to the PDCP SN of the next data packet expected to bereceived in the PDCP status transmitted to an eNB during the enhancedhandover process. If the handover is not an enhanced handover, theT-SeNB knows the HFN corresponding to the PDCP SN of the next datapacket expected to be received according to the existing principle.

For the split bearer, the MeNB can know the receive status of the UE,and thus transmit corresponding data to the T-SeNB.

In order to solve the problem in the uplink data transmission in thepresent invention, according to the uplink receive status and the uplinkcount received in the message of the step 908 b and in combination withthe received uplink data forwarded by the S-SeNB, the T-SeNB updates theuplink receive status and then transmits the updated uplink receivestatus to the UE. The T-SeNB needs to form a new uplink receive statusafter all the forwarded data has been received from the S-SeNB. TheT-SeNB knows that the S-SeNB has forwarded all the data according to anend marker. The T-SeNB transmits the generated uplink receive status tothe UE. According to the uplink receive status received from the T-SeNBand in combination with the data transmitted at the S-SeNB by the UE andthe feedback received from the S-SeNB, the UE transmits uplink data tothe T-SeNB starting from the next data packet that has not been receivedby the S-SeNB. This method is specific to an SCG bearer.

For a split bearer, the steps 908 a and 908 b can be executed at anytime after the step 903.

Step 910: By the MeNB, a UE context release request message istransmitted to the S-SeNB.

The detailed descriptions of steps irrelevant to the main contents ofthe present application are omitted here, for example, the ERABmodification indication process between the MeNB and the MME.

Now, the embodiment for dual connectivity of the fourth and fifthmethods for supporting seamless handover provided by the presentapplication has been described. By this method, the interruption time ofdata transmission during the handover process can be reduced, thecontinuous transmission of uplink and downlink data can be ensured, andthe missing or duplication transmission of data can be avoided.Particularly, the problem that the T-SeNB does not know an HFNcorresponding to the next PDCP SN expected by the UE is solved. Thus,even if the MeNB, the S-SeNB and the T-SeNB are from differentmanufacturers, the interoperability between the eNBs is ensured, and theinterruption time of data transmission during the handover process isreduced while ensuring the operability during the enhanced handoverprocess.

The SeNB addition process in the TS36.300 10.1.2.8.1 can also be used bythe MeNB to decide to adopt an enhanced handover process. A specificmethod comprises the following steps.

1) The MeNB decides to adopt an enhanced handover process, and informsan SeNB through an SeNB addition request message.

2) The SeNB contains enhanced handover indication information in acontainer from the SeNB to the MeNB of an SeNB addition requestacknowledge message, and then transmits this message to a UE by theMeNB.

3) The MeNB transmits an RRC reconfiguration request message to the UE,this message containing the enhanced handover indication information.

4) The UE knows this handover is an enhanced handover process inaccordance with the step 3).

The steps focus on the method for deciding to adopt the enhancedhandover process, and the data transmission, data receipt and dataforwarding of eNBs are similar to those in FIGS. 7 and 9 and will not bedescribed in details here.

The SeNB modification process in the TS36.300 10.1.2.8.2 can also beused by the MeNB to decide to adopt an enhanced handover process. Aspecific method comprises the following steps.

1) The MeNB decides to adopt an enhanced handover process, and informsan SeNB through an SeNB modification request message.

2) The SeNB contains enhanced handover indication information in acontainer from the SeNB to the MeNB of an SeNB modification requestacknowledge message, and then transmits this message to a UE by theMeNB.

3) The MeNB transmits an RRC reconfiguration request message to the UE,this message containing the enhanced handover indication information.

4) The UE knows this handover is an enhanced handover process inaccordance with the step 3).

The internal handover of the MeNB in the TS36-300 10.1.2.8.2.1 involvesan SCG change process, and can also be used by the MeNB to decide toadopt an enhanced handover process. A specific method is the same as theabove method and will not be repeated there.

The steps focus on the method for deciding to adopt the enhancedhandover process, and the data transmission, data receipt and dataforwarding of eNBs are similar to those in FIGS. 7 and 9 and will not bedescribed in details here.

The SeNB release process in the TS36.300 10.1.2.8.3 can also be used bythe MeNB to decide to adopt an enhanced handover process. A specificmethod comprises the following steps.

1) The MeNB decides to adopt an enhanced handover process, and theninforms an SeNB through an SeNB release request message (an SeNB releaseprocess initiated by the MeNB) or an SeNB release acknowledge (an SeNBrelease process initiated by the SeNB).

2) The MeNB transmits enhanced handover indication information to a UE.

3) The MeNB transmits an RRC reconfiguration request message to the UE,this message containing the enhanced handover indication information.

4) The UE knows this handover is an enhanced handover process inaccordance with the step 3).

The steps focus on the method for deciding to adopt the enhancedhandover process, and the data transmission, data receipt and dataforwarding of eNBs are similar to those in FIGS. 7 and 9 and will not bedescribed in details here.

The change process from an MeNB to an eNB in the TS36.300 10.1.2.8.5 canalso be used by a source MeNB to decide to adopt an enhanced handoverprocess. A specific method comprises the following steps.

1) The source MeNB decides to adopt an enhanced handover process, andinforms a target eNB through a handover request message.

2) The target eNB contains enhanced handover indication information inan RRC container of a handover request acknowledge message, and thentransmits this message to a UE by the source MeNB.

3) The source MeNB informs an S-SeNB of the enhanced handover indicationinformation through an SeNB release request. The S-SeNB continuestransmitting data to the UE and receiving uplink data from the UE.

4) The source MeNB transmits an RRC reconfiguration request message tothe UE, this message containing the enhanced handover indicationinformation.

5) The UE knows this handover is an enhanced handover process inaccordance with the step 4).

The steps focus on the method for deciding to adopt the enhancedhandover process, and the data transmission, data receipt and dataforwarding of eNBs are similar to those in FIGS. 7 and 9 and will not bedescribed in details here.

The change process from an eNB to an MeNB in the TS36.300 10.1.2.8.7 canalso be used by a source eNB to decide to adopt an enhanced handoverprocess. A specific method comprises the following steps.

1) The source eNB decides to adopt an enhanced handover process, andinforms a target MeNB through a handover request message.

2) The target MeNB informs a T-SeNB of the enhanced handover indicationinformation through an SeNB addition request message.

3) The T-SeNB contains the enhanced handover indication information inan RRC container of an SeNB addition request acknowledge message. TheT-SeNB contains the enhanced handover indication information in an RRCcontainer of a handover request acknowledge message, and then transmitsthis message to a UE via the source eNB. Or, the T-SeNB directlycontains the enhanced handover indication information in the RRCcontainer of the handover request acknowledge message, and does notcontain the enhanced handover indication information in the RRCcontainer of the SeNB addition request acknowledge message.

4) The source eNB transmits an RRC reconfiguration request message tothe UE, this message containing the enhanced handover indicationinformation.

5) The UE knows this handover is an enhanced handover process inaccordance with the step 4).

The steps focus on the method for deciding to adopt the enhancedhandover process, and the data transmission, data receipt and dataforwarding of eNBs are similar to those in FIGS. 7 and 9 and will not bedescribed in details here.

A scenario of performing handover between different MeNBs withoutchanging an SeNB in the TS36.300 10.1.2.8.8 can also be used by a sourceMeNB to decide to adopt an enhanced handover process. A specific methodis a combination of the change process from an MeNB to an eNB in theTS36.300 10.1.2.8.5 and the change process from an eNB to an MeNB in theTS36.300 10.1.2.8.7, and will not be repeated here.

FIG. 10 shows a sixth method for supporting seamless handover accordingto the present invention. This method comprises the following steps.

Step 1001: By a source eNB, a handover request message is transmitted toa target eNB.

The message contains the capability for supporting the enhanced handoverof a UE. The capability for supporting the enhanced handover of the UEmeans that, upon receiving the RRC connection reconfiguration message,the UE may not freeze the transmission and receive status at the sourceeNB and not reset or empty the information at the layer 2 (L2) of thesource eNB, and may continue receiving or transmitting data from thesource eNB. The specific meaning of the capability for supporting theenhanced handover of the UE is the same as that in the step 701 and willnot be repeated here. The capability for supporting the enhancedhandover of the UE can be contained in a Radio Resource Control (RRC)container. The source eNB can also contain an information element(supporting the enhanced handover process) in the handover requestmessage. By using this information element, the source eNB informs thetarget eNB that the source eNB supports the enhanced handover; or, byusing this information element, the source eNB informs the target eNBthat both the source eNB and the UE support the enhanced handoverprocess. The meaning of the enhanced handover process is the same asthat in the step 701 and will not be repeated here.

Step 1002: By the target eNB, a handover request acknowledge message istransmitted to the source eNB.

In this method, the target eNB decides to adopt an enhanced handoverprocess. The target eNB decides to adopt the enhanced handover processaccording to its support for the enhanced handover process and thecapability of the UE. Or, the target eNB decides to adopt the enhancedhandover process according to its support for the enhanced handoverprocess, the capability of the UE, and the capability for supporting theenhanced handover process of the source eNB. The target eNB can alsodecide to adopt the enhanced handover process by taking the requirementsfor a service (e.g., Quality of service (QoS) information) intoconsideration. The target eNB can also decide to adopt the enhancedhandover process by taking other information into consideration, withoutinfluencing the main contents of the present invention. The target eNBacquires, from the UE, the capability for supporting the enhancedhandover of the UE. The target eNB knows the capability for supportingthe enhanced handover process of the source eNB through an Operation &Maintenance (O&M) configuration or an X2 establishment process.Corresponding to the a method using the X2 establishment process, bothan X2 establishment request message and an X2 establishment responsemessage contain the capability for supporting the enhanced handoverprocess of an eNB transmitting the message. An eNB receiving the X2establishment request message and the X2 establishment response messagemaintains the received capability for supporting the enhanced handoverprocess of the opposite eNB. Corresponding to the method of informing,by the source eNB, the target eNB that both the source eNB and the UEsupport the enhanced handover process by containing an informationelement of supporting the enhanced handover process in the handoverrequest message in the step 1001, the target eNB can know, through thereceived information element, that both the source eNB and the UEsupport the enhanced handover process.

In the present invention, there are following methods to indicate to thesource eNB that an enhanced handover process is adopted.

Method 1: By containing enhanced handover indication information in thehandover request acknowledge message, the target eNB indicates to thesource eNB to adopt the enhanced handover process. The enhanced handoverindication information is also contained in an RRC container from thetarget eNB to the source eNB, and is transmitted to the UE by the sourceeNB.

Method 2: The enhanced handover indication information is contained inan RRC container. Upon receiving the handover request acknowledgemessage, the source eNB informs and views the RRC container and thusknows this handover process is an enhanced handover process. The sourceeNB transmits the RRC container to the UE, so that the UE is indicatedthat this handover is an enhanced handover process.

Step 1003: By the source eNB, an RRC connection reconfiguration messageis transmitted to a UE. This message contains the enhanced handoverindication information. Corresponding to the enhanced handover process,the source eNB continues transmitting downlink data to the UE.Corresponding to a method provided by the present invention, the datapackets continuously transmitted to the UE by the source eNB are at mostthe number of data packets within the maximum PDCP SN range minus 1starting from the first data packet forwarded to the target eNB. Forexample, if the first data packet forwarded to the target eNB by thesource eNB has a PDCP SN of 5 and an HFN of 10, the data packetstransmitted to the UE by the source eNB at most have a PDCP SN of 3 andan HFN of 11. According to the enhanced handover indication informationreceived in the step 1002 or by viewing the enhanced handover indicationinformation in the RRC container, the source eNB knows that the enhancedhandover process is adopted.

The source eNB continues receiving uplink data from the UE. Uponreceiving this message, the UE continues receiving the downlink datatransmitted by the source eNB, and continues transmitting the uplinkdata at the source eNB. According to the enhanced handover indicationinformation contained in the received RRC connection reconfigurationmessage, the UE knows that this handover process is an enhanced handoverprocess, so that the UE continues transmitting and receiving data at thesource eNB.

In order to solve the problem in the uplink data transmission in thepresent invention, one corresponding method is that the source eNBcontinues feeding an uplink data receive status back to the UE afterthis step or after step 1004. Thus, the UE knows which data packets havebeen already received from the UE by the source eNB. Accordingly, upontransmitting a message in step 1006, the UE knows a data packet startingfrom which the data packet should be transmitted to the target eNB.

Step 1004: By the source eNB, a Sequence Number (SN) status transfer istransmitted to the target eNB.

The source eNB can inform the target eNB of the maximum number of datapackets transmitted to the UE starting from the first data packetforwarded to the target eNB during the enhanced handover process, forexample, the number of data packets within the maximum PDCP SN rangeminus 1, or more. For example, if the PDCP SN is from 0 to 127, the datapackets within the maximum PDCP SN range minus 1 are 127 PDCP SDUs orPDUs.

In this step, the source eNB does not freeze the transmission andreceive status.

The source eNB starts to forward data to the target eNB. The source eNBalso forwards, to the target eNB, the downlink data that is transmittedto the UE.

The source eNB forwards, to the target eNB, the uplink data packetsreceived from the UE. One method is as follows: the source eNB forwards,to the target eNB, all the received uplink packets including the datapackets received in-sequence or out-of-sequence after the SN statustransfer message. Another method is as follows: the source eNB transmitthe received data packets in sequence an SGW, and forwards the receiveddata packets out of sequence to the target eNB.

Step 1005: The UE is synchronized to the target eNB. The UE accesses toa target cell via a Random Access Channel (RACH).

In the present invention, the UE can stop receiving the downlink datafrom the source eNB and stop transmitting the uplink data at the sourceeNB in this step. Or, the UE can stop receiving the downlink data fromthe source eNB and stop transmitting the uplink data at the source eNBin step 1006.

In order to solve the problem in the uplink data transmission in thepresent invention, according to the uplink receive status and the uplinkcount received in the message of the step 1004 and in combination withthe uplink data that is forwarded by the source eNB and received fromthe source eNB, the target eNB updates the uplink receive status andthen transmits the updated uplink receive status to the UE. If thetarget eNB still continues receiving the data forwarded from the sourceeNB after the RACH is successful, the target eNB forms a new uplinkreceive status upon receiving all forwarded data from the source eNB.The target eNB knows that the source eNB has finished data forwardingaccording to an end marker.

Step 1006: By the UE, an RRC connection reconfiguration completionmessage is transmitted to the target eNB. After the step 1003, thesource eNB simultaneously transmits the downlink data to the UE andforwards the downlink data to the target eNB. Therefore, a part of theforwarded data received from the source eNB by the target eNB may bealready received by the UE. There are following ways for the target eNBto detect data that don't need to be transmitted to the UE.

According to the PDCP status report received from the UE, the target eNBknows the next PDCP SN expected to be received by the UE. The target eNBdetects duplicated data packets that do not need to be transmitted tothe UE, and discards the data that has been received by the UE, andtransmits data packet to the UE directly starting from the next datapacket expected by the UE.

The PDCP status report transmitted to the target eNB by the UE merelycontains the PDCP SN of the next data packet expected to be received,but does not contain an HFN corresponding to the PDCP SN.

In the step 1002, the target eNB has decided that this handover adoptsthe enhanced handover process. For the enhanced handover process, thetarget eNB transmits data to the UE and/or transmits uplink data to anSGW by the method for the enhanced handover process. The target eNBdetermines an HFN corresponding to the PDCP SN of the next data packetexpected to be received by the UE according to the enhanced handoverprocess.

The target eNB considers that the data packet corresponding to the nextexpected PDCP SN contained in the PDCP status received from the UE is afirst data packet corresponding to this PDCP SN stored in a buffer ofthe target eNB. The maximum number of data packets transmitted to the UEby the source eNB starting from the first data packet forwarded to thetarget eNB is the number of data packets within the maximum PDCP SNrange minus 1. For example, if the first data packet received from thesource eNB by the target eNB has a PDCP SN of 5 and an HFN of 10, thedata packets transmitted to the UE by the source eNB at most have a PDCPSN of 3 and an HFN of 11, so the maximum data packet received from thesource eNB by the UE has a PDCP SN of 3 and an HFN of 11. If the datatransmitted by the source eNB all has been received by the UE, the UEexpects that the next received data packet has a PDCP SN of 4 and an HFNof 11. Therefore, this data packet is a first corresponding data packetto having a PDCP SN of 4 in the buffer of the target eNB, and the HFNcorresponding to this data packet is 11. If some data packetstransmitted by the source eNB have not been received by the UE, forexample, if the next expected PDCP SN in the PDCP status received fromthe UE by the target eNB is 9, this data packet is a first correspondingdata packet having a PDCP SN of 9 in the buffer of the target eNB, andthe target eNB knows that the corresponding HFN is 10. Therefore, ifthere are data packets having a same PDCP SN in the buffer of the targeteNB, a first data packet not receiving a data packet SN in the firstcorresponding UE PDCP status report is the next data packet to betransmitted to the UE. In this way, the target eNB knows an HFNcorresponding to this data packet.

Or, the target eNB considers that the next expected PDCP SN contained inthe PDCP status received from the UE is not far from a half of the PDCPSN range of the PDCP SN in the DL count received in the SN statustransfer message. In other words, if the maximum value of the PDCP SN isN, the next expected PDCP SN is not far from N/2 of the PDCP SN in theDL count. The source eNB also follows this principle, when transmittingdata to the UE after transmitting the SN status transfer message. Forexample:

-   -   in the DL count, the HFN is 10, and the PDCP SN is 8;    -   the PDCP SN is 7 bits (the range of the SN is from 0 to 127);    -   when the next expected PDCP SN received from the UE by the        target eNB is 73 to 127, the HFN is 9; and    -   when the next expected PDCP SN received from the UE by the        target eNB is 0 to 72, the HFN is 10.

Upon transmitting the SN status transfer message, the source eNBcontinues transmitting the data to the UE, and then stops transmittingdownlink data packets to the UE after the PDCP SN is 72 and the HFN is10.

Corresponding to the enhanced handover process, if the next expectedPDCP SN in the PDCP status received from the UE by the target eNB is 9,the target eNB knows that the corresponding HFN is 10. If the handoveris not an enhanced handover, the target eNB knows that the correspondingHFN is 9.

In order to solve the problem in the uplink data transmission in thepresent invention, according to the uplink receive status and the uplinkcount received in the message of the step 1004 and in combination withthe uplink data that is forwarded by the source eNB and received fromthe source eNB, the target eNB updates the uplink receive status andthen transmits the updated uplink receive status to the UE. The targeteNB needs to form a new uplink receive status after all the forwardeddata has been received from the source eNB. The target eNB knows thatthe source eNB has finished data forwarding according to an end marker.The target eNB transmits the generated uplink receive status to the UE.

According to the uplink receive status received from the target eNB andin combination with the data transmitted by the UE at the source eNB andthe feedback received from the source eNB, the UE transmits uplink datato the target eNB starting from the next data packet that has not beenreceived by the source eNB.

Step 1007: By the target eNB, a path handover request message istransmitted to an MME.

Step 1008: By the MME, a path handover request response message istransmitted to the target eNB.

Step 1009: By the target eNB, a UE context release message istransmitted to the source eNB.

Now, the sixth method for supporting seamless handover provided by thepresent invention has been described. By this method, the interruptiontime of data transmission during the handover process can be reduced,the continuous transmission of uplink and downlink data can be ensured,and the missing or duplication transmission of data can be avoided.Particularly, the problem that the target eNB does not know an HFNcorresponding to the next PDCP SN expected by the UE is solved. Thus,even if the source eNB and the target eNB are from differentmanufacturers, the interoperability between the two eNBs is ensured, andthe interruption time of data transmission during the handover processis reduced while ensuring the operability during the enhanced handoverprocess.

FIG. 11 is a schematic diagram of an embodiment for dual connectivity ofthe sixth method for supporting seamless handover according to thepresent invention. This method comprises the following steps.

Step 1101: By a Master eNB (MeNB), a Secondary eNB (SeNB) additionrequest message is transmitted to a Target SeNB (T-SeNB).

The message contains the capability for supporting the enhanced handoverof a UE. The capability for supporting the enhanced handover of the UEmeans that the UE supports a handover process for maintaining aconnection to a source eNB as discussed in the 3GPP Rel-14.Specifically, the capability for supporting the enhanced handover of theUE means that, upon receiving an RRC connection reconfiguration message,the UE may not freeze the transmission and receive status at the sourceeNB and not reset or empty the information at the layer 2 (L2) of anS-SeNB, and may continue receiving or transmitting data from the S-SeNB.More specifically, the specific meaning of the capability for supportingthe enhanced handover of the UE is the same as that in the step 701 andwill not be repeated here. The capability for supporting the enhancedhandover of the UE can be contained in a Radio Resource Control (RRC)container. Here, the RRC container is a container from the MeNB to theSeNB. The MeNB can also contain an information element (supporting theenhanced handover process) in the SeNB addition request message. Byusing this information element, the MeNB informs the T-SeNB that theMeNB supports the enhanced handover; or, by using this informationelement, the MeNB informs the T-SeNB that both the MeNB and the UEsupport the enhanced handover process. The meaning of the enhancedhandover process is the same as that in the step 901 and will not berepeated here.

Step 1102: By the T-SeNB, an SeNB addition request acknowledge messageis transmitted to the MeNB.

In this method, the T-SeNB decides to adopt an enhanced handoverprocess. The T-SeNB decides, according to the capability of the UE, thecapability for supporting capability for supporting the enhancedhandover of the MeNB, and the capability for supporting the enhancedhandover of the S-SeNB and/or the T-SeNB, whether to adopt the enhancedhandover. The T-SeNB can also decide to adopt the enhanced handoverprocess by taking other information (e.g., QoS) into consideration,without influencing the main contents of the present invention. TheT-SeNB acquires, from the MeNB, the capability for supporting theenhanced handover of the UE. The T-SeNB knows the capability forsupporting the enhanced handover process of the MeNB and the S-SeNBthrough an Operation & Maintenance (O&M) configuration or an X2establishment process. Corresponding to the a method using the X2establishment process, both an X2 establishment request message and anX2 establishment response message contain the capability for supportingthe enhanced handover process of an eNB transmitting the message. An eNBreceiving the X2 establishment request message and the X2 establishmentresponse message maintains the received capability for supporting theenhanced handover process of the opposite eNB. Corresponding to themethod of informing, by the MeNB, the T-SeNB that both the MeNB and theUE support the enhanced handover process by containing an informationelement of supporting the enhanced handover process in the SeNB additionrequest message in the step 1101, the T-SeNB can know, through thereceived information element, that both the MeNB and the UE support theenhanced handover process.

In the present invention, there are following methods to indicate to theMeNB that an enhanced handover process is adopted.

Method 1: By containing enhanced handover indication information in theSeNB addition request acknowledge message, the T-SeNB indicate to theMeNB to adopt an enhanced handover process. The enhanced handoverindication is also contained in a container from the SeNB to the MeNB,and is transmitted to the UE by the MeNB.

Method 2: The enhanced handover indication information is contained in acontainer from the SeNB to the MeNB. Upon receiving the SeNB additionrequest acknowledge message, the MeNB informs and views the containerfrom the SeNB to the MeNB and thus knows this handover process is anenhanced handover process. The MeNB transmits, to the UE, the containerfrom the SeNB to the MeNB, so that the UE is indicated that thishandover process is an enhanced handover process.

Step 1103: If resources for the T-SeNB are allocated successfully, bythe MeNB, an SeNB release request message is transmitted to a SourceSeNB (S-SeNB). If the data is to be forwarded, the MeNB provides theS-SeNB with a data forwarding address.

In the present invention, there are following methods to indicate theS-SeNB that the MeNB adopts the enhanced handover process.

Method 1: By containing the enhanced handover indication information inthe SeNB release request message, the MeNB indicates to the S-SeNB thatthis handover process is an enhanced handover process. The MeNB knowsthat this process is an enhanced handover process in accordance with thestep 1102.

Method 2: The capability for supporting the enhanced handover of the UEis contained in the SeNB addition release message. If the S-SeNBsupports the enhanced handover process, the S-SeNB knows, according tothe capability for supporting the enhanced handover process of the MeNBand/or the T-SeNB and the capability for supporting the enhancedhandover of the UE, knows that this handover process is an enhancedhandover process. The MeNB acquires, from the UE, the capability forsupporting the enhanced handover of the UE. The S-SeNB knows thecapability for supporting the enhanced handover process of the MeNBand/or the T-SeNB through an Operation & Maintenance (O&M) configurationor an X2 establishment process. Corresponding to the a method using theX2 establishment process, both an X2 establishment request message andan X2 establishment response message contain the capability forsupporting the enhanced handover process of an eNB transmitting themessage. An eNB receiving the X2 establishment request message or the X2establishment response message maintains the received capability forsupporting the enhanced handover process of the opposite eNB.Corresponding to this method, if it is assumed that the MeNB, the S-SeNBand the T-SeNB all support the enhanced handover process, an enhancedhandover process will be initiated when a handover to a UE supportingthe enhanced handover is initiated.

Upon receiving the SeNB release message, the S-SeNB continuestransmitting data to the UE. The S-SeNB can start to forward the data.The SeNB continues receiving uplink data from the UE.

Corresponding to a split bearer, the S-SeNB continues transmittinguplink data to the MeNB. Corresponding to a Second Cell Group (SCG)bearer, the S-SeNB continues transmitting uplink data to an SGW.

For an SCG bearer, the S-SeNB continues feeding an uplink data receivestatus to the UE after this step. Thus, the UE knows which data packetshave been already received from the UE by the S-SeNB. Accordingly, upontransmitting a message in step 1107, the UE knows a data packet startingfrom which the data packet should be transmitted to the T-SeNB.

Step 1104: By the MeNB, an RRC connection reconfiguration message istransmitted to a UE. This message contains the enhanced handoverindication information. The MeNB continues transmitting downlink data tothe S-SeNB. Meanwhile, the MeNB forwards the downlink data to theT-SeNB. Corresponding to a method provided by the present invention, thedata packets continuously transmitted to the UE by the S-SeNB are atmost the number of data packets within the maximum PDCP SN range minus 1starting from the first data packet forwarded to the T-SeNB. Forexample, if the first data packet forwarded to the T-SeNB by the S-SeNBhas a PDCP SN of 5 and an HFN of 10, the data packets transmitted to theUE by the S-SeNB at most have a PDCP SN of 3 and an HFN of 11.

The UE receives the RRC connection reconfiguration message. The UE doesnot delete or reset the configuration for the S-SeNB. The UE continuesreceiving the downlink data transmitted by the S-SeNB. The UE continuestransmitting the uplink data to the S-SeNB. According to the enhancedhandover indication information contained in the received RRC connectionreconfiguration message, the UE knows that this handover process is anenhanced handover process, so that the UE continues transmitting andreceiving data at the source eNB.

Step 1105: By the UE, an RRC connection reconfiguration completionmessage is transmitted to the MeNB.

Step 1106: By the MeNB, an SeNB reconfiguration completion message istransmitted to the T-SeNB.

Step 1107: The UE is synchronized to the T-SeNB.

The UE stops receiving the downlink data from the S-SeNB, and also stopstransmitting the uplink data to the S-SeNB.

Step 1108 a: By the S-SeNB, an SN status transfer is transmitted to theMeNB.

Step 1108 b: By the MeNB, the SN status transfer is transmitted to theT-SeNB.

Step 1109: By the S-SeNB, the data is forwarded to the T-SeNB.Corresponding to the split bearer, the S-SeNB forwards the data to theMeNB, and then the MeNB forwards the data to the T-SeNB. For the SCGbearer, like the split bearer, the S-SeNB can forward the data to theT-SeNB via the MeNB, or the S-SeNB can directly forward the data to theT-SeNB. Depending on the implementation, the forwarding the data to theT-SeNB by the S-SeNB can be executed at any time after the step 1103.

After the step 1103, the S-SeNB simultaneously transmits the downlinkdata to the UE and forwards the downlink data to the T-SeNB. Therefore,a part of the forwarded data received from the S-SeNB by the T-SeNB maybe already received by the UE. The T-SeNB detects data that is not to betransmitted to the UE in the following ways.

For the SCG bearer, according to the PDCP status report received fromthe UE, the T-SeNB knows the next PDCP SN expected to be received by theUE. The target eNB detects duplicated data packets that do not need tobe transmitted to the UE, and discards the data that has been receivedby the UE, and transmits data packet to the UE directly starting fromthe next data packet expected by the UE.

For the SCG bearer, the PDCP status report transmitted to the T-SeNB bythe UE merely contains the PDCP SN of the next data packet expected tobe received, but does not contain an HFN corresponding to the PDCP SN.There are following two ways for the T-SeNB to know an HFN correspondingto the PDCP SN of the next data packet expected to be received.

Way 1:

By the method described in the step 1101, the T-SeNB can know whetherthis handover process is an enhanced handover process. For the enhancedhandover process, the T-SeNB transmits data to the UE and/or transmitsuplink data to the SGW by the method for the enhanced handover process.

The T-SeNB considers that the data packet corresponding to the nextexpected PDCP SN contained in the PDCP status received from the UE is afirst data packet corresponding to this PDCP SN stored in a buffer ofthe T-SeNB. The maximum number of data packets transmitted to the UE bythe S-SeNB starting from the first data packet forwarded to the T-SeNBis the number of data packets within the maximum PDCP SN range minus 1.For example, if the first data packet received from the S-SeNB by theT-SeNB has a PDCP SN of 5 and an HFN of 10, the data packets transmittedto the UE by the S-SeNB at most have a PDCP SN of 3 and an HFN of 11.Therefore, the maximum data packet received from the S-SeNB by the UEhas a PDCP SN of 3 and an HFN of 11. If the data transmitted by theS-SeNB all has been received by the UE, the UE expects that the nextreceived data packet has a PDCP SN of 4 and an HFN of 11. Therefore,this data packet is a first corresponding data packet to having a PDCPSN of 4 in the buffer of the T-SeNB, and the HFN corresponding to thisdata packet is 11. If some data packets transmitted by the S-SeNB havenot been received by the UE, for example, if the next expected PDCP SNin the PDCP status received from the UE by the T-SeNB is 9, this datapacket is a first corresponding data packet having a PDCP SN of 9 in thebuffer of the T-SeNB, and the T-SeNB knows that the corresponding HFN is10. Therefore, if there are data packets having a same PDCP SN in thebuffer of the T-SeNB, a first data packet not receiving a data packet SNin the first corresponding UE PDCP status report is the next data packetto be transmitted to the UE. In this way, the T-SeNB knows an HFNcorresponding to this data packet.

Way 2:

The PDCP status report transmitted to the T-SeNB by the UE contains thePDCP SN of the next data packet expected to be received and an HFNcorresponding to the PDCP SN. The UE can merely contain an HFNcorresponding to the PDCP SN of the next data packet expected to bereceived in the PDCP status transmitted to an eNB during the enhancedhandover process. If the handover is not an enhanced handover, theT-SeNB knows the HFN corresponding to the PDCP SN of the next datapacket expected to be received according to the existing principle.

For the split bearer, the MeNB can know the receive status of the UE,and thus transmit corresponding data to the T-SeNB.

In order to solve the problem in the uplink data transmission in thepresent invention, according to the uplink receive status and the uplinkcount received in the message of the step 1108 b and in combination withthe received uplink data forwarded by the S-SeNB, the T-SeNB updates theuplink receive status and then transmits the updated uplink receivestatus to the UE. The T-SeNB needs to form a new uplink receive statusafter all the forwarded data has been received from the S-SeNB. TheT-SeNB knows that the S-SeNB has forwarded the data according to an endmarker. The T-SeNB transmits the generated uplink receive status to theUE. According to the uplink receive status received from the T-SeNB andin combination with the data transmitted at the S-SeNB by the UE and thefeedback received from the S-SeNB, the UE transmits uplink data to theT-SeNB starting from the next data packet that has not been received bythe S-SeNB. This method is specific to an SCG bearer.

For a split bearer, the steps 1108 a and 1108 b can be executed at anytime after the step 903.

Step 1110: By the MeNB, a UE context release request message istransmitted to the S-SeNB.

The detailed descriptions of steps irrelevant to the main contents ofthe present application are omitted here, for example, the ERABmodification indication process between the MeNB and the MME.

Now, the embodiment for dual connectivity of the fourth and fifthmethods for supporting seamless handover provided by the presentapplication has been described. By this method, the interruption time ofdata transmission during the handover process can be reduced, thecontinuous transmission of uplink and downlink data can be ensured, andthe missing or duplication transmission of data can be avoided.Particularly, the problem that the T-SeNB does not know an HFNcorresponding to the next PDCP SN expected by the UE is solved. Thus,even if the MeNB, the S-SeNB and the T-SeNB are from differentmanufacturers, the interoperability between the eNBs is ensured, and theinterruption time of data transmission during the handover process isreduced while ensuring the operability during the enhanced handoverprocess.

The SeNB addition process in the TS36.300 10.1.2.8.1 can also be used bythe SeNB to decide to adopt an enhanced handover process. A specificmethod comprises the following steps.

1) The SeNB decides to adopt an enhanced handover process.

2) The SeNB contains enhanced handover indication information in acontainer from the SeNB to an MeNB of an SeNB addition requestacknowledge message, and then transmits this message to a UE by theMeNB.

3) There are following two ways for the MeNB to know that an enhancedhandover process is adopted. Way 1: the MeNB parses the enhancedhandover indication information in the container from the SeNB to theMeNB in the SeNB addition request acknowledge message, and thus knowsthis handover process is an enhanced handover process. Way 2: The SeNBalso contains the enhanced handover indication information in the SeNBaddition request acknowledge message. According to the enhanced handoverindication information in the SeNB addition request acknowledge message,without parsing the RRC container, the MeNB can know that the SeNBdecides to adopt the enhanced handover process.

4) The MeNB transmits an RRC reconfiguration request message to a UE,this message containing the enhanced handover indication information.

5) The UE knows this handover process is an enhanced handover process inaccordance with the step 4).

The steps focus on the method for deciding to adopt the enhancedhandover process, and the data transmission, data receipt and dataforwarding of eNBs are similar to those in FIGS. 10 and 11 and will notbe described in details here.

The SeNB modification process initiated by an MeNB and an SeNB in theTS36.300 10.1.2.8.2 can also be used by the SeNB to decide to adopt anenhanced handover process. A specific method comprises the followingsteps.

1) The SeNB decides to adopt an enhanced handover process.

2) The SeNB contains enhanced handover indication information in acontainer from the SeNB to the MeNB of an SeNB modification requestacknowledge message, and then transmits this message to a UE via theMeNB.

3) There are following two ways for the MeNB to know that an enhancedhandover process is adopted. Way 1: The MeNB parses the enhancedhandover indication information in the container from the SeNB to theMeNB in the SeNB modification request acknowledge message, and thusknows this handover process is an enhanced handover process. Way 2: TheSeNB also contains the enhanced handover indication information in theSeNB modification request acknowledge message. According to the enhancedhandover indication information in the SeNB modification requestacknowledge message, without parsing the RRC container, the MeNB canknow that the SeNB decides to adopt the enhanced handover process.

4) The MeNB transmits an RRC reconfiguration request message to a UE,this message containing the enhanced handover indication information.

5) The UE knows this handover process is an enhanced handover process inaccordance with the step 4).

The steps focus on the method for deciding to adopt the enhancedhandover process, and the data transmission, data receipt and dataforwarding of eNBs are similar to those in FIGS. 10 and 11 and will notbe described in details here.

The SeNB modification process initiated by an SeNB in the TS36.30010.1.2.8.2 can also be used by the SeNB to decide to adopt an enhancedhandover process. A specific method comprises the following steps.

1) The SeNB decides to adopt an enhanced handover process.

2) The SeNB contains enhanced handover indication information in acontainer from the SeNB to an MeNB of an SeNB modification requestacknowledge message, and then transmits this message to a UE by theMeNB.

3) There are following two ways for the MeNB to know that an enhancedhandover process is adopted. Way 1: The MeNB parses the enhancedhandover indication information in the container from the SeNB to theMeNB in the SeNB modification request acknowledge message, and thusknows this handover process is an enhanced handover process. Way 2: TheSeNB also contains the enhanced handover indication information in theSeNB modification request acknowledge message meanwhile. According tothe enhanced handover indication information in the SeNB modificationrequest acknowledge message, without parsing the RRC container, the MeNBcan know that the SeNB decides to adopt the enhanced handover process.

4) The MeNB transmits an RRC reconfiguration request message to a UE,this message containing the enhanced handover indication information.

5) The UE knows this handover process is an enhanced handover process inaccordance with the step 4).

The internal handover of the MeNB in the TS36-300 10.1.2.8.2.1 involvesan SCG change process, and can also be used by the SeNB to decide toadopt an enhanced handover process. A specific method is the same as theabove method and will not be repeated there.

The steps focus on the method for deciding to adopt the enhancedhandover process, and the data transmission, data receipt and dataforwarding of eNBs are similar to those in FIGS. 10 and 11 and will notbe described in details here.

The change process from an MeNB to an eNB in the TS36.300 10.1.2.8.5 canalso be used by the target eNB to decide to adopt an enhanced handoverprocess. A specific method comprises the following steps.

1) The target eNB decides to adopt an enhanced handover process.

2) The target eNB contains enhanced handover indication information inan RRC container of a handover request acknowledge message, and thentransmits this message to a UE by a source MeNB.

3) There are following two ways for the source MeNB to know that anenhanced handover process is adopted. Way 1: The source MeNB parses theenhanced handover indication information in the RRC container in thehandover request acknowledge message, and thus knows this handoverprocess is an enhanced handover process. Way 2: The target eNB alsocontains the enhanced handover indication information in the handoverrequest acknowledge message. According to the enhanced handoverindication information in the handover request acknowledge message,without parsing the RRC container, the source MeNB can know that thetarget eNB decides to adopt the enhanced handover process.

4) The source MeNB informs an S-SeNB of the enhanced handover indicationinformation through an SeNB release request. The S-SeNB continuestransmitting data to the UE and receiving uplink data from the UE.

5) The source MeNB transmits an RRC reconfiguration request message tothe UE, this message containing the enhanced handover indicationinformation.

6) The UE knows this handover process is an enhanced handover process inaccordance with the step 5).

The steps focus on the method for deciding to adopt the enhancedhandover process, and the data transmission, data receipt and dataforwarding of eNBs are similar to those in FIGS. 10 and 11 and will notbe described in details here.

The change process from an eNB to an MeNB in the TS36.300 10.1.2.8.7 canalso be used by a target MeNB to decide to adopt an enhanced handoverprocess. A specific method comprises the following steps.

1) The target MeNB decides to adopt an enhanced handover process. Thetarget MeNB decides to adopt the enhanced handover process uponreceiving a handover request message.

2) The target MeNB informs a T-SeNB of the enhanced handover indicationinformation through an SeNB addition request message.

3) The T-SeNB contains the enhanced handover indication information inan RRC container of an SeNB addition request acknowledge message. Thetarget MeNB contains the enhanced handover indication information in anRRC container of the handover request acknowledge message, and thentransmits this message to a UE via a source eNB. Or, the target MeNBdirectly contains the enhanced handover indication information in theRRC container of the handover request acknowledge message, and does notcontain the enhanced handover indication information in the RRCcontainer of the SeNB addition request acknowledge message.

4) The source eNB transmits an RRC reconfiguration request message tothe UE, this message containing the enhanced handover indicationinformation.

5) The UE knows this handover process is an enhanced handover process inaccordance with the step 4).

The steps focus on the method for deciding to adopt the enhancedhandover process, and the data transmission, data receipt and dataforwarding of eNBs are similar to those in FIGS. 10 and 11 and will notbe described in details here.

The change process from an eNB to an MeNB in the TS36.300 10.1.2.8.7 canalso be used by a T-SeNB to decide to adopt an enhanced handoverprocess. A specific method comprises the following steps.

1) The T-SeNB decides to adopt an enhanced handover process. The targetMeNB decides to adopt the enhanced handover process upon receiving anSeNB addition request message.

2) The T-SeNB contains the enhanced handover indication information inan RRC container of an SeNB addition request acknowledge message. Thetarget MeNB contains the enhanced handover indication information in anRRC container of a handover request acknowledge message, and thentransmits this message to a UE via a source eNB. Or, the T-SeNB informsthe target MeNB of the enhanced handover indication information throughthe SeNB addition request acknowledge message, and the target MeNBcontains the enhanced handover indication information in the RRCcontainer of the handover request acknowledge message while the T-SeNBdoes not contain the enhanced handover indication information in the RRCcontainer of the SeNB addition request acknowledge message. There arefollowing two ways to indicate the source eNB that an enhance handoverprocess is adopted.

Way 1: The source eNB parses the enhanced handover indicationinformation in the RRC container of the handover request acknowledgemessage, and thus knows this handover process is an enhanced handoverprocess.

Way 2: By containing the enhanced handover indication information in anX2 access layer message of the SeNB addition request acknowledge and thehandover request acknowledge, without parsing the RCC container in thehandover request acknowledge message, the source eNB is indicated thatan enhanced handover process is adopted.

3) The source eNB transmits an RRC reconfiguration request message to aUE, this message containing the enhanced handover indicationinformation.

4) The UE knows this handover process is an enhanced handover process inaccordance with the step 3).

The steps focus on the method for deciding to adopt the enhancedhandover process, and the data transmission, data receipt and dataforwarding of eNBs are similar to those in FIGS. 10 and 11 and will notbe described in details here.

A scenario of performing handover between different MeNBs withoutchanging an SeNB in the TS36.300 10.1.2.8.8 can also be used by a targetMeNB to decide to adopt an enhanced handover process. A specific methodis a combination of the change process from an MeNB to an eNB in theTS36.300 10.1.2.8.5 and the change process from an eNB to an MeNB in theTS36.300 10.1.2.8.7, and will not be repeated here.

Corresponding to the first specific embodiment, an embodiment of thepresent invention further provides an eNB equipment which is a sourceeNB, comprising: a first transmitting module, a first processing moduleand a first receiving module, wherein:

the first transmitting module is configured to: transmit a handoverrequest message to a target eNB; transmit an RRC connectionreconfiguration message to a UE, and continue transmitting downlink datato the UE; and, transmit an SN status transfer to the target eNB, andstart to forward data to the target eNB;

the first receiving module is configured to: continue receiving uplinkdata from the UE; receive a data transmission stopping indicationtransmitted after the UE is synchronized to the target eNB and indicatethe first processing module to perform processing; and, receive a UEcontext release message transmitted by the target eNB; and

the first processing module is configured to, under an indication fromthe receiving module, control the first transmitting module to stoptransmitting the downlink data to the UE and control the first receivingmodule to stop receiving the uplink data from the UE.

Corresponding to the first specific embodiment, an embodiment of thepresent invention further provides an eNB equipment which is a targeteNB, comprising: a second transmitting module and a second receivingmodule, wherein:

the second receiving module is configured to: receive a handover requestmessage transmitted by a source eNB; receive an RRC connectionreconfiguration message transmitted to a UE by the source eNB; receivean RRC connection reconfiguration completion message transmitted by theUE; and receive a path handover request response message transmitted byan MME; and

the second transmitting module is configured to: transmit a handoverrequest acknowledge message to the source eNB; transmit, by a targeteNB, a data transmission stopping indication to the source eNB after theUE is synchronized to the target eNB; transmit a path request message tothe MME; and transmit a UE context release message to the source eNB.

Corresponding to the second specific embodiment, an embodiment of thepresent invention further provides an eNB equipment which is a sourceeNB, comprising: a third transmitting module, a third processing moduleand a third receiving module, wherein:

the third transmitting module is configured to: transmit a handoverrequest message to a target eNB; transmit an RRC connectionreconfiguration message to a UE, and continue transmitting downlink datato the UE; and, transmit an SN status transfer to the target eNB, andstart to forward data to the target eNB;

the third receiving module is configured to: receive a handover requestacknowledge message transmitted by the target eNB, and continuereceiving uplink data from the UE; receive a data transmission stoppingindication transmitted by the target eNB after receiving an RRCconnection reconfiguration completion message and indicates the thirdprocessing module to perform processing; and, receive a UE contextrelease message transmitted by the target eNB; and

the third processing module is configured to, under an indication fromthe third receiving module, control the transmitting module to stoptransmitting the downlink data to the UE and control the third receivingmodule to stop receiving the uplink data from the UE.

Corresponding to the second specific embodiment, an embodiment of thepresent invention further provides an eNB equipment which is a targeteNB, comprising: a fourth transmitting module and a fourth receivingmodule, wherein:

the fourth transmitting module is configured to: transmit a handoverrequest acknowledge message to a source eNB; transmit a datatransmission stopping indication to the source eNB; transmit a pathhandover request message to an MME; and transmit a UE context releasemessage to the source eNB; and

the fourth receiving module is configured to: receive a handover requestmessage transmitted by the source eNB; receive an RRC connectionreconfiguration message transmitted to the UE by the source eNB; receivean RRC connection reconfiguration completion message transmitted by theUE; and receive a path handover request response message transmitted bythe MME.

The foregoing description merely shows preferred embodiments of thepresent invention and is not intended to limit the present invention.Any modification, equivalent replacement or improvement made within thespirit and principle of the present invention shall fall into theprotection scope of the present invention.

What is claimed is:
 1. A method performed by a first base station in acommunication system, the method comprising: transmitting, to a secondbase station, a handover request message for a user equipment (UE);receiving, from the second base station, a handover request acknowledgemessage as a response to the handover request message; transmitting, tothe UE, a radio resource configuration (RRC) reconfiguration messageassociated with a handover for the UE; transmitting, to the second basestation, a first status transfer message associated with data forwardingto the second base station; receiving, from the second base station, acontrol message associated with stopping of data transmission to the UE,the control message including an identifier of the UE associated withthe second base station and an identifier of the UE associated with thefirst base station; and transmitting, to the second base station, as aresponse to the control message, a second status transfer messageincluding packet data convergence protocol (PDCP) status information,wherein the data transmission to the UE is continued after transmissionof the RRC reconfiguration message.
 2. The method of claim 1, whereinthe PDCP status information includes at least one of: an uplink PDCPsequence number (SN) and hyper frame number (HFN); or a downlink PDCP SNand HFN.
 3. The method of claim 1, wherein transmitting the handoverrequest message further comprise: receiving, from the UE, UE capabilityinformation including information indicating the UE supports an enhancedhandover process; and transmitting, to the second base station, thehandover request message including information associated with theenhanced handover process based on the UE capability information.
 4. Themethod of claim 3, wherein the handover request acknowledge messageincludes an RRC container and first information indicating that theenhanced handover process is adopted, wherein the RRC container includessecond information indicating that the enhanced handover is adopted, andwherein the RRC reconfiguration message includes the RRC container. 5.The method of claim 1, wherein data reception from the UE is continuedafter transmission of the RRC reconfiguration message, and wherein thedata transmission to the UE is not stopped after transmission of thefirst status transfer message.
 6. A method performed by a second basestation in a communication system, the method comprising: receiving,from a first base station, a handover request message for a userequipment (UE); transmitting, to the first base station, a handoverrequest acknowledge message as a response to the handover requestmessage; receiving, from the first base station, a first status transfermessage associated with data forwarding to the second base station;transmitting, to the first base station, a control message associatedwith stopping of data transmission to a user equipment (UE), the controlmessage including an identifier of the UE associated with the secondbase station and an identifier of the UE associated with the first basestation; and receiving, from the first base station, as a response tothe control message, a second status transfer message including packetdata convergence protocol (PDCP) status information.
 7. The method ofclaim 6, wherein the PDCP status information includes at least one of:an uplink PDCP sequence number (SN) and hyper frame number (HFN); or adownlink PDCP SN and HFN.
 8. The method of claim 6, wherein the handoverrequest message includes information associated with an enhancedhandover process, wherein transmitting the handover request acknowledgemessage further comprise transmitting, to the first base station, thehandover request acknowledge message including a radio resource control(RRC) container and first information indicating that the enhancedhandover process is adopted, in case that the enhanced handover processis adopted, and wherein the RRC container includes second informationindicating that the enhanced handover is adopted.
 9. The method of claim6, further comprising: performing synchronization with the UE afterreception of the first status transfer message; and receiving, from theUE, an RRC reconfiguration complete message.
 10. The method of claim 6,further comprising: transmitting, to a mobility management entity, apath switch request message; receiving, from the mobility managemententity, as a response to the path switch request message, a path switchrequest acknowledge message; and transmitting, to the first basestation, a UE context release message.
 11. A first base station in acommunication system, the first base station comprising: a transceiver;and a controller coupled with the transceiver and configured to:transmit, to a second base station, a handover request message for auser equipment (UE), receive, from the second base station, a handoverrequest acknowledge message as a response to the handover requestmessage, transmit, to the UE, a radio resource configuration (RRC)reconfiguration message associated with a handover for the UE, transmit,to the second base station, a first status transfer message associatedwith data forwarding to the second base station, receive, from thesecond base station, a control message associated with stopping of datatransmission to the UE, the control message including an identifier ofthe UE associated with the second base station and an identifier of theUE associated with the first base station, and transmit, to the secondbase station, as a response to the control message, a second statustransfer message including packet data convergence protocol (PDCP)status information, wherein the data transmission to the UE is continuedafter transmission of the RRC reconfiguration message.
 12. The firstbase station of claim 11, wherein the PDCP status information includesat least one of: an uplink PDCP sequence number (SN) and hyper framenumber (HFN); or a downlink PDCP SN and HFN.
 13. The first base stationof claim 11, wherein the controller is further configured to: receive,from the UE, UE capability information including information indicatingthe UE supports an enhanced handover process, and transmit, to thesecond base station, the handover request message including informationassociated with the enhanced handover process based on the UE capabilityinformation.
 14. The first base station of claim 13, wherein thehandover request acknowledge message includes an RRC container and firstinformation indicating that the enhanced handover process is adopted,wherein the RRC container includes second information indicating thatthe enhanced handover is adopted, and wherein the RRC reconfigurationmessage includes the RRC container.
 15. The first base station of claim11, wherein data reception from the UE is continued after transmissionof the RRC reconfiguration message, and wherein the data transmission tothe UE is not stopped after transmission of the first status transfermessage.
 16. A second base station in a communication system, the secondbase station comprising: a transceiver; and a controller coupled withthe transceiver and configured to: receive, from a first base station, ahandover request message for a user equipment (UE), transmit, to thefirst base station, a handover request acknowledge message as a responseto the handover request message, receive, from the first base station, afirst status transfer message associated with data forwarding to thesecond base station, transmit, to the first base station, a controlmessage associated with stopping of data transmission to a userequipment (UE), the control message including an identifier of the UEassociated with the second base station and an identifier of the UEassociated with the first base station, and receive, from the first basestation, as a response to the control message, a second status transfermessage including packet data convergence protocol (PDCP) statusinformation.
 17. The second base station of claim 16, wherein the PDCPstatus information includes at least one of: an uplink PDCP sequencenumber (SN) and hyper frame number (HFN); or a downlink PDCP SN and HFN.18. The second base station of claim 16, wherein the handover requestmessage includes information associated with an enhanced handoverprocess, wherein transmitting the handover request acknowledge messagefurther comprise transmitting, to the first base station, the handoverrequest acknowledge message including a radio resource control (RRC)container and first information indicating that the enhanced handoverprocess is adopted, in case that the enhanced handover process isadopted, and wherein the RRC container includes second informationindicating that the enhanced handover is adopted.
 19. The second basestation of claim 16, wherein the controller is further configured to:perform synchronization with the UE after reception of the first statustransfer message; and receive, from the UE, an RRC reconfigurationcomplete message.
 20. The second base station of claim 16, wherein thecontroller is further configured to: transmit, to a mobility managemententity, a path switch request message, receive, from the mobilitymanagement entity, as a response to the path switch request message, apath switch request acknowledge message, and transmit, to the first basestation, a UE context release message.